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1 upporting Information Reversible Generation of Metastable Enols in the,-addition of Thioacetic Acid to α,β- Unsaturated Carbonyl Compounds Lukas Hintermann*, and Aleksej Turočkin Contents: General Reactions of carbonyl compounds with thioacetic acid. Reaction of acrolein with thioacetic acid. Reaction of crotonaldehyde with thoacetic acid. Reaction of (E)--hexenal with thioacetic acid in [D ]acetone. Reaction of MVK (-buten--one) with thioacetic acid in [D ]acetone. Reaction of -penten--one with thioacetic acid in [D ]acetone 7. Hemithioacetal from butanal and thioacetic acid 8

2 General. Chemicals Crotonaldehyde ((E)--butenal), acrolein (propenal) and -buten--one were purified via bulb-to-bulb distillation before use. The -penten--one was a technical grade and contained 0 mol% of mesityl oxide (-methyl-- buten--one; by H NMR spectroscopy). ther chemicals were used as received from commercial suppliers. All substances were handled in air. Thioacetic acid hydrolyzes over time, generating acetic acid, which is the main impurity. A rough distillation cannot remove acetic acid completely, but the presence of acetic acid is of no particular concern when generating simple enols, as it does not efficiently catalyze tautomerization (thioacetic acid, pk a =.0; vs acetic acid, pk a =.). We have initially used an aged brand which contained 0 mol% of HAc (by H NMR) after distillation, but later found it more convenient to use freshly ordered commercial thioacetic acid straight from the bottle, which contained % or less of acetic acid. An earlier paper claimed two chemical shifts for the methyl group of thioacetic acid in CDCl between 0 and 0 C (τ = 7. and 7.9, ; i.e. δ =.8 and.0 ppm); the minor peak (9±%) was thought to be that of the tautomer CH (C=)H. We (and others) have never observed a separate peak for the thiono-tautomer, but find that acetic acid impurities (δ =.0 ppm, CDCl ) are very common and difficult to remove from thioacetic acid (δ =.0 ppm, CDCl ).. NMR spectroscopy NMR spectra were recorded at ambient temperature ( 9 K). A relaxation delay (d) of 0 seconds was used for quantitative integration. As internal standard, CH Cl was added to most samples, although this compound is not necessarily ideal for all measurements in terms of the spectral window, and its content slowly diminishes by preferential evaporation in long-term experiments. Crosschecks and, if necessary, corrections were performed by integrating (immediately after mixing) over specific sets of signals (e.g., methyl groups in crotonaldehyde and its adducts) and equaling the sum to 00% of the added carbonyl compound. Chemical shifts δ are given in ppm. The positional numbering scheme for individual compounds is defined ad hoc and need not correspond to that derived from systematic nomenclature. Referencing of the spectra was performed with TM as internal standard for samples recorded in CDCl, acetone-d, C D and DM-d. For methanol-d or mixtures of D and methanol-d, the residual signal of methanol was used as reference. Coupling constants n J are given in Hz. The following abbreviations were used to describe the signals in the NMR spectra: s singlet, d doublet, t triplet, q quartet, quint quintet, sext sextet, sept septet and m complex multiplet. ψ (pseudo) indicates that a signal appears (by coincidence) like a regular multiplet (e.g. ψ-t = pseudo triplet). NMR signals for corresponding nuclei in diastereomer mixtures are separated by a slash /, unless shift values could be clearly assigned to single diastereomers. W. P. Jencks, K. alvesen, J. Am. Chem. oc. 97, 9,. R. Taylor, J. Chem. oc. Perkin Trans. 98, 89.

3 . General procedure A solution of the Michael acceptor and CH Cl (internal standard) in deuterated solvent was analyzed by H and C-NMR spectroscopy. Thioacetic acid was then added by microsyringe and the reaction mixture monitored by NMR spectroscopy.. Accuracy of integration and recovery ummation over the mol-% composition of reaction components, (E)-, (Z)-, and x should ideally reproduce the amount in mol-% of initially added thioacetic acid (; typically 0 mol-%), whereas summation over, (E)-, (Z)-, and should ideally reproduce the initial amount of carbonyl compound (00 mol-%). In fact, the recovery is usually lower (8 00%, but often 90%), which is due to losses of material by either: polymerization of carbonyl compound ; generation of other, non-identified reaction products; presence of acetic acid (up to 0%) as impurity in thioacetic acid The latter aspect is responsible for relatively low recoveries of ; the content of acetic acid increases over the reaction due to hydrolysis of thioacetic acid; note also the comments under. concerning the methodology, scope and limitations of the H NMR integration results. n the other hand, the reaction of thioacetic acid with acetone-d to reversibly form a hemithioacetal does not affect the integration, since this is a fast equilibrium at room temperature that gives only rise to a single timeaveraged signal for thioacetic acid. T. Horii,. Kawamura, J. Tsurugi, Bull. Chem. oc. Jpn. 97,, 00.

4 Reactions of carbonyl compounds with thioacetic acid NMR data of thioacetic acid: H-NMR (0 MHz, [D ]acetone, 9 K): δ =. (s, H, H-),.0 (br s, H, H). H-NMR (0 MHz, CDCl, 9 K): δ =.0 (s, H, H-),.88 (br s, H, H). H-NMR (00 MHz, C D, 9 K): δ =. (s, H, H-),. (br. s, H, H). H-NMR (0 MHz, H [D ]DM, 9 K): δ =. (s, H, H-),?; signal not detected (s, H, H); not stable. C-NMR ( MHz, [D ]acetone, 9 K): δ =. (CH, C-), 9. (CH, C-). C-NMR ( MHz, CDCl, 9 K): δ =. (CH, C-), 9. (CH, C-). C-NMR ( MHz, C D, 9 K): δ =.9 (CH, C-), 9. (CH, C-). C-NMR (9 MHz, [D ]DM, 9 K): δ = 9. (CH, C-), 90. (CH, C-); not stable.. Reaction of acrolein with thioacetic acid.. Reaction of acrolein and thioacetic acid in [D ]acetone etup: Acrolein (7 µl, 0. mmol), [D ]acetone (00 µl), CH Cl ( µl), thioacetic acid ( µl, 0. mmol). H H a (Z)-a (E)-a H Ac a AcH Ac a H Example spectrum: recorded 0 min after mixing ( H NMR, 0 MHz, [D ]acetone) Composition after 0 min: (7%), a (.9%), (Z)-a (9.%), (E)-a (.%), a (.0%), a (9.%); HAc (%). [Recovery: thioacetic (including acetic) acid: 0 of 08 mol-%; of acrolein: 9. of 00 mol-%.]

5 H, C-HQC of an acrolein/thioacetic acid reaction mixture in [D ]acetone (00 MHz) Notes: Compare Figure in the main text and numerical data below for assignments. The cross-peak at δ( H) =. ppm / δ( C) = 7.9 ppm is assigned to H-/C- of the minor component a. H and C traces were recorded at 0 MHz and 90 MHz, respectively. elected NMR data of reaction mixture components: Acrolein (a): H-NMR (0 MHz, [D ]acetone, 9 K): δ =. (ddd, J = 7., 9., 7. Hz, H H, H-),.8 (dd, J = 7. Hz, J =. Hz, H, H-),. (dd, J = 9. Hz, J =. Hz, H, H- ), 9.0 (d, J = 7. Hz, H, H-). C-NMR (90 MHz, [D ]acetone, 9 K): δ = 8. (CH, C- H ' H H ), 9. (CH, C-), 9. (CH, C-). (Z)--Acetylsulfanyl--propen--ol, (Z)-a: H-NMR (00 MHz, [D ]acetone, 9 K): δ =.8 (s, H, H-),.8 (d, J = 8. Hz, H, H-),. (td, J = 8.0 Hz, J =. Hz, H, H-),. (ψt, J =. Hz, H, H-), 7.7 (d, J =.9 Hz, H, H). C-NMR (90 MHz, [D ]acetone, 9 K): H δ =. (CH, C-),.0 (CH, C-), 99. (CH, C-),. (CH, C-), 9. (CMe, C-). (E)--Acetylsulfanyl--propen--ol, (E)-a: H-NMR (00 MHz, [D ]acetone, 9 K): δ =. (s, H, H-),. (d, J = 7.9 Hz, H, H-),.79 (dt, J =. Hz, J = 7.9 Hz, H, H-),.7 (dd, J =. Hz, J = 7.8 Hz, H, H-), 7. (d, J = 9. Hz, H, H). C-NMR (90 MHz, [D ]acetone, H 9 K): δ = 8. (CH, C-), 0. (CH, C-), 99.9 (CH, C-),.7 (CH, C-), 9. (CMe, C-).

6 -Acetylsulfanyl-propanal (Michael adduct, a): H-NMR (0 MHz, [D ]acetone, 9 K): δ =.79 (td, J =.7 Hz, J = 0.8 Hz, H, H-),.09 (t, J =.8 Hz, H, H-), 9.7 (t, J =.0 Hz, H, H-). C-NMR (90 MHz, [D ]acetone, 9 K): δ =.8 (CH, C-),.0 (CH, C-), 9. (CMe, C-), 00.8 (CH, C-).,-Bis(acetylsulfanyl)--propanol (hemithioacetal of Michael adduct, a): H-NMR (0 MHz, [D ]acetone, 9 K): δ =.9. (m, H, H-),.. (m, H, H- and H-7),.9.0 (m, H, H-),. (d, J =. Hz, H, H),.9.9 (m, H, H-). C-NMR H (90 MHz, [D ]acetone, 9 K): δ =.0 (CH, C-), 0. (CH, C- or C-7),. (CH, C- or C-7), 8.0 (CH, C-), 7.9 (CH, C-), 9. (CMe, C- or C-), 9.0 (CMe, C- or C-). 7.. Reaction of acrolein and thioacetic acid in [D ]DM etup: Acrolein (7 µl, 0. mmol), [D ]DM (00 µl), CH Cl ( µl), ( µl, 0. mmol). H H a (Z)-a (E)-a H Ac a The reaction in DM quickly produces the enols, and is then successively depleted from excess thioacetic acid (δ =. ppm) by the latter s irreversible chemical reaction with DM to give acetic acid (δ =.9 ppm). Hemithioacetal a only made a short appearance (after 0 min), whereas Michael adduct a was prominent (besides the enols a) after h. After prolonged reaction times (> h), the concentration of acrolein (which had sunk to <% after 0 min) rose again to >0% (> d), whereas the enols had disappeared. Evidently, the enols a had reverted to acrolein and thioacetic acid, and the latter was removed from the equilibrium by reacting irreversibly with DM; a was the major reaction product after d (7%). Example spectrum: recorded 0 min after mixing ( H NMR, 0 MHz, [D ]DM) Composition after 0 min: (.7%), a (0.%), (Z)-a (.0%), (E)-a (70.0%), a (.%), a (.%)

7 7 elected NMR data of reaction mixture components: Acrolein (a): H-NMR (0 MHz, [D ]DM, 9 K): δ =. (ddd, J = 7., 9.7, 7. Hz, H, H-),.9 (dd, J = 7. Hz, J =. Hz, H, H-),. (dd, J = 9.7 Hz, J =. Hz, H, H- ), 9.7 (d, J = 7. Hz, H). C-NMR (90 MHz, [D ]DM, 9 K): δ = 8. (CH, C-), 9. (CH, C-), 9. (CH, C-). (Z)--Acetylsulfanyl--propen--ol, (Z)-a: H-NMR (0 MHz, [D ]DM, 9 K): δ =.9 H H ' H H (s, H, H-),. (dd, J = 7.9 Hz, J = 0.9 Hz, H, H-),. (td, J = 7.9 Hz, J =. Hz, H, H-),..9 (m, H, H-), 8.8 (d, J =.9 Hz, H, H). C-NMR (90 MHz, [D ]DM, H 9 K): δ = 97. (CH, C-),.9 (CH, C-), 9.7 (CMe, C-). (E)--Acetylsulfanyl--propen--ol, (E)-a: H-NMR (0 MHz, [D ]DM, 9 K): δ =.9 (s, H, H-),. (dd, J = 7.9 Hz, J = 0.8 Hz, H, H-),. (dt, J =. Hz, J = 7.9 Hz, H, H-),.. (m, H, H-), 8. (d, J =.9 Hz, H, H). C-NMR (90 MHz, H [D ]DM, 9 K): δ = 98. (CH, C-),. (CH, C-), 9. (CMe, C-). -Acetylsulfanyl-propanal (Michael adduct, a): H-NMR (0 MHz, [D ]DM, 9 K): δ =. (s, H, H-),.7 (t, J =.8 Hz, H, H-),.0 (t, J =.7 Hz, H, H-), 9. (s, H, H-). C-NMR (90 MHz, [D ]DM, 9 K): δ =. (CH, C-), 0. (CH, C-).,-Bis(acetylsulfanyl)--propanol (hemithioacetal of Michael adduct, a): H-NMR (0 MHz, [D ]DM, 9 K): δ =.8.0 (m, H, H-),.87.9 (m, H, H-),..0 (m, H, H-). 7 H.. Reaction of acrolein and thioacetic acid in [D ]MeH etup: Acrolein (7 µl, 0. mmol), [D ]MeH (00 µl), CH Cl ( µl), thioacetic acid ( µl, 0. mmol). D CD D CD D CD D Ac CD H D D Ac AcD D D D a (Z)-a (E)-a a Ac D Note: olutions of acrolein (a) in [D ]methanol displayed signals for a hemiacetal with methanol (K eq L mol ; measured after h standing at r.t.). A trace of -methyl-,-dioxolan was also detected, which appears to be an impurity of the commercial acrolein.

8 Example spectrum: recorded min after mixing ( H NMR, 0 MHz, [D ]methanol): 8 Composition after min: (7.%), a (0.9%), (Z)-a (.%), (E)-a (7.%), a (.0%), a (8.%), acrolein hemiacetal with methanol (.%) elected NMR data of reaction mixture components: Acrolein (a): H-NMR (00 MHz, [D ]methanol, 9 K): δ =. (ddd, J = 7., 9., 7. Hz, H H, H-),. (dd, J = 7. Hz, J =. Hz, H, H-),. (dd, J = 9. Hz, J =. Hz, H, H- ), 9. (d, J = 7. Hz, H, H-). C-NMR ( MHz, [D ]methanol, 9 K): δ = 9. (CH, C-), 9. (CH, C-), 9. (CH, C-). -Methoxy--propen--ol (acrolein methanol hemiacetal): H NMR (00 MHz, [D ]methanol, 9 K): δ =.9 (d, J =.7 Hz, H-),.9 (d m, J = 0. Hz, H- ),. (d m, J = 7. Hz, H-),.8 (ddd, J = 7., 0.,.7 Hz, H-) ppm. C NMR ( MHz, [D ]methanol, 9 K): δ =. (t, J CD = Hz, CD ), 98. (C-), 7. (C-), 8. (C-). (Z)--Acetylsulfanyl--propen--ol, (Z)-a: H-NMR (00 MHz, [D ]methanol, 9 K): δ =.9 (s, H, H-),.7 (dd, J = 8.0 Hz, J =.0 Hz, H, H-),. (td, J = 8.0 Hz, J =. Hz, H, H-),.7 (d, J =. Hz, H, H-). C-NMR ( MHz, [D ]methanol, 9 K): δ =. (CH, C-), 0. (CH, C-), 99. (CH, C-),. (CH, C-), 98.7 (CMe, C-). (E)--Acetylsulfanyl--propen--ol, (E)-a: H-NMR (00 MHz, [D ]methanol, 9 K): δ =.8 (s, H, H-),. (dd, J = 8.0 Hz, J =.0 Hz, H, H-),.79 (dt, J =. Hz, J = 8.0 Hz, H, H-),.9 (d, J =. Hz, H, H-). C-NMR ( MHz, [D ]methanol, 9 K): δ = 9. (CH, C-), 0. (CH, C-), 00. (CH, C-),.9 (CH, C-), 97.8 (CMe, C-). H ' H H H H ' H CD H H D D

9 9 Additional components at prolonged reaction times included hemi-thioacetals of the Michael adduct with either methanol or thioacetic acid, which were not completely assigned... Reaction of acrolein and thioacetic acid in [D ]MeH/D etup: Acrolein (7 µl, 0. mmol), [D ]MeH/D : (v/v) (00 µl), CH Cl ( µl), thioacetic acid ( µl, 0. mmol). D a H D (Z)-a (E)-a D Ac a D D Ac AcD Ac D D D D a Example spectrum: recorded min after mixing ( H NMR, 0 MHz, [D ]methanol) Composition after min: (9.%), a (0%), (Z)-a (.%), (E)-a (7.%), a (0%), a (9.%), hydrate of a (.7%). elected NMR data of reaction mixture components: Acrolein (a): H-NMR (0 MHz, [D ]methanold, 9 K): δ =.9 (ddd, J = 7., 9., 7.7 H Hz, H, H-),.0 (dd, J = 7. Hz, J =. Hz, H, H-),. (dd, J = 9., J =. Hz, H, H- ), 9. (d, J = 7.7 Hz, H, H-). C-NMR (90 MHz, [D ]methanold, 9 K): δ = 8.9 H ' H H (CH, C-),. (CH, C-), 98. (CH, C-).

10 0 (Z)--Acetylsulfanyl--propen--ol, (Z)-a: H-NMR (0 MHz, [D ]methanold, 9 K): δ =. (dd, J = 8.0 Hz, J =.0 Hz, H, H-),.8 (td, J = 8.0 Hz, J =. Hz, H, H-),. (dt, J =. Hz, J =. Hz, H, H-). (E)--Acetylsulfanyl--propen--ol, (E)-a: H-NMR (0 MHz, [D ]methanol D, 9 K): δ =. (dd, J = 7.9 Hz, J =.0 Hz, H, H-),.8 (dt, J =. Hz, 7.9 Hz, H, H-),. (dt, J =. Hz, J =. Hz, H, H-). C-NMR (90 MHz, [D ]methanold, 9 D D K): δ = 9.0 (C, C- or C, C-), 00.7 (CH, C-),. (CH, C-). -Acetylsulfanyl-propan-,-diol, hydrate of Michael adduct: H-NMR (0 MHz, [D ]methanold, 9 K): δ =.9 (d, J = 7. Hz, H, H-),. (d, J =. Hz, H, H- ). C-NMR (90 MHz, [D ]methanold, 9 K): δ =. (C, C- or C-),. (C, C- or C-),. (CD, J CD = 0 Hz), 98. (CH, C-), 99.8 (CMe, C-). D D,-Bis-acetylsulfanyl--propanol, hemithioacetal of Michael adduct (a): H-NMR (0 MHz, [D ]methanold, 9 K): δ =.9.0 (m, H, H-). (s, H, H- or H-7),. (s, H, H- or H-7),.9.00 (m, H, H-),.7 (d, J =.9 Hz, H, H-). C-NMR (90 MHz, [D ]methanold, 9 K): δ = 0.8 (C, C- or C-7),. (C, C- or C-7),.7 (CD, J CD = 0 Hz, C-), 7.7 (CH, C-), 99.0 (CMe, C- or C-), 99. (CMe, C- or C-). 7 D D

11 . Reaction of crotonaldehyde with thioacetic acid.. Reaction of crotonaldehyde in [D ]acetone etup: Crotonaldehyde ( µl, 0. mmol), [D ]acetone (00 µl), CH Cl ( µl), thioacetic acid ( µl, 0. mmol). K H eq H AcH H b (Z)-b b b H, C-HQC (00 MHz, [D ]acetone) Note: The HQC was recorded in the absence of CH Cl as internal standard, whereas the D trace for the H NMR from a different experiment contains the CH Cl peak. Besides the major crosspeaks due to enolic species, correlations for b are also visible.

12 Example spectrum: recorded h after mixing ( H NMR, 00 MHz, [D ]acetone) Composition after 0 min: (0%), (Z)-b (.%), (E)-b (.9%), (Z)-b (0.%), (E)-b (0.%), b (0.%), b (.%). elected spectral data of reaction components: Crotonaldehyde (b): H-NMR (00 MHz, [D ]acetone, 9 K): δ =.00 (dd, J =.8 Hz, J =. Hz, H, H-),.09 (ddq, J =. Hz, J = 7.9 Hz, J =. Hz, H, H-),.98 (dq, J =. Hz, J =.8 Hz, H, H-), 9.9 (d, J = 7.9 Hz, H, H-). C-NMR ( MHz, [D ]acetone, 9 K): δ = 8.7 (CH, C-),. (CH, C-),.8 (CH, C-), 9. (CH, C-). A small amount of (Z)-crotonaldehyde is also present during the reaction (δ = 0. ppm, d, J = 8.0 Hz, H-). H (Z)--Acetylsulfanyl--buten--ol (Z)-b: H-NMR (00 MHz, [D ]acetone, 9 K): δ =. (d, J =.9 Hz, H, H-),. (s, H, H-),. (dd, J = 0.0 Hz, J =. Hz, H, H-),. (dq, J = 0.0 Hz, J =.8 Hz, H, H-),. (ψ-t, J =. Hz, H, H-), 7.70 (d, J =.8 Hz, H, H). C-NMR ( MHz, [D ]acetone, 9 K): δ =. (CH, C-), H.8 (CH, C-), 0. (CH, C-),. (CH, C-), 9.8 (CMe, C-). (E)--Acetylsulfanyl--buten--ol (E)-b: H-NMR (00 MHz, [D ]acetone, 9 K): δ =. (d, J = 7. Hz, H, H-),. (s, H, H-),.0 (m, H, H-),.8 (dd, J =. Hz, J = 8.7 Hz, H, H-),. (dd, J =. Hz, J = 8.8 Hz, H, H-), 7.7 (d, J = 9. Hz, H, H). Amount too small for obtaining reliable C NMR data. H

13 -Acetylsulfanyl-butanal (b): H-NMR (00 MHz, [D ]acetone, 9 K): δ =.7 (dt, J =. Hz, J =.7 Hz, H, H-),.9 (sext, J =.9 Hz, H, H-), 9. (t, J =. Hz, H, H- ). C-NMR ( MHz, [D ]acetone, 9 K): δ =. (CH, C-),.9 (CH, C-), 0. (CH, C-), 9.9 (CMe, C-), 00. (CH, C-). H,-Bis(acetylsulfanyl)--butanol (b), hemithioacetal of b, mixture of diastereomers: H-NMR (00 MHz, [D ]acetone, 9 K): δ =. (dd, J = 8.9 Hz, J =.9 Hz, H, H-),.90.9/.07. (m, H, H-),.. (m, H, H- 8 and H-8),..7 (m, H, H-),. (br s, H, H),. (br s, H, H-). C- H NMR ( MHz, [D ]acetone, 9 K): δ = 0./0.7 (CH, C-), 0./0. (CH, C- or C-8), 0.9/.0 (CH, C- or C-8),./7. (CH, C-),./. (CH, C-), 7.9/7.0 (CH, C-), 9.7/9.0 (CMe, C- or C- 7), 9.9/9.0 (CMe, C- or C-7). 7.. Reaction of crotonaldehyde in C D etup: Crotonaldehyde ( µl, 0. mmol), C D (00 µl), CH Cl (8 µl), thioacetic acid (7 µl, 0. mmol). K H eq H AcH H b (Z)-b b b Example spectrum: recorded 0 min after mixing ( H NMR, 00 MHz, C D ) Composition after 0 min: (89%), (Z)-b (.%), (E)-b (.%), (Z)-b (.%), (E)-b (0%), b (0%), b (.%).

14 elected spectral data of reaction components: Crotonaldehyde (b): H-NMR (00 MHz, C D, 9 K): δ =.7 (dd, J =.7 Hz, J =. Hz, H, H-),.8 (ddq, J =. Hz, J = 7.8 Hz, J =. Hz, H, H-),.9 (dq, J =. Hz, J =.7 Hz, H, H-), 9. (d, J = 7.8 Hz, H, H-). C-NMR ( MHz, C D, 9 K): H δ = 8.0 (CH, C-),.7 (CH, C-),. (CH, C-), 9.7 (CH, C-). A small amount of (Z)- crotonaldehyde (δ = 9.80 ppm, d, J = 8. Hz, H-) is also present during the reaction. (Z)--Acetylsulfanyl--buten--ol (Z)-b: H-NMR (00 MHz, C D, 9 K): δ =.09 (d, J = 7. Hz, H, H-),. (dd, J = 0.0 Hz, J =. Hz, H, H-),.9 (dq, J = 0.0 Hz, J = 7. Hz, H, H-),. (d, J =. Hz, H, H-), 7.8 (br s, H, H). C- NMR ( MHz, C D, 9 K): δ = 0.7 (CH, C-), 9.8 (CH, C-),.9 (CH, C-), 07.7 (CH, C-),.7 (CH, C-), 00. (CMe, C-). H -Acetylsulfanyl-butanal (b): H-NMR (00 MHz, C D, 9 K): δ =.0 (d, J = 7.0 Hz, H, H-),.8 (s, H, H-),.80.9 (m, H, H-), (m, H, H-). C-NMR ( MHz, C D, 9 K): δ =.9 (CH, C-), 0. (CH, C-),. (CH, C-), 9.8 (CH, C-), 9.8 (C, C-), 98.8 (CH, C-). H,-Bis(acetylsulfanyl)--butanol (b), hemithioacetal of b (mixture of diastereomers): H-NMR (00 MHz, C D, 9 K): δ =.7 (d, J =.8 Hz, H, H-), (m, H, H- and H-8),.9.07/.. (m, H, H-), (m, H, H-),.7.79 (m, H, H-). C-NMR ( MHz, C D, 9 K): δ = 0.8/0.9 (CH, C-), 0./0. (CH, C- or C-8), 0.8/0.9 (CH, C- or C- 8),./.7 (CH, C-),.8/.9 (CH, C-), 7./7.8 (CH, C-), 9./9. (CMe, C- or C-7), 98./98. (CMe, C- or C-7). H. Reaction of (E)--hexenal with thioacetic acid in [D ]acetone etup: (E)--Hexenal ( µl, 0. mmol), [D ]acetone (00 µl), CH Cl (8 µl), thioacetic acid (7 µl, 0. mmol). K H eq H n-pr n-pr n-pr AcH n-pr H c (Z)-c c c

15 Example spectrum: recorded min after mixing ( H NMR, 0 MHz, [D ]acetone) Composition after 0 min: (%), (E)-c (8.%), (Z)-c (.%), (E)-c (.%), c (0%), c (0.7%). elected spectral data of reaction mixture components: (E)--Hexenal (c): H-NMR (0 MHz, [D ]acetone, 9 K): δ = 0.9 (t, J = 7. Hz, H, H- ),. (sext, J = 7. Hz, H, H-),.0.7 (m, H, H-),.07 (ddt, J =. Hz, J = 7.9 Hz, J =. Hz, H, H-),.97 (dt, J =. Hz, J =.8 Hz, H, H-), 9. (d, J = 7.9 Hz, H, H-). C- NMR (90 MHz, [D ]acetone, 9 K): δ =.0 (CH, C-),.0 (CH, C-),. (CH, C-),.0 (CH, C- ), 9.8 (CH, C-), 9. (CH, C-). (Z)--Acetylsulfanyl--hexen--ol (Z)-c: H-NMR (0 MHz, [D ]acetone, 9 K): δ = 0.90 (t, J = 7. Hz, H, H-),.. (m, H, H-),.7 (s, H, H-8),. (dd, J = 0. Hz, J =. Hz, H, H-),.. (m, H, H-),.8 (ψ-t, J =. Hz, H, H-), 7.0 (d, J = 7. H Hz, H, H). C-NMR (90 MHz, [D ]acetone, 9 K): δ =.8 (CH, C-),.0 (CH, C-), 0. (CH, C-8), 9.0 (CH, C-), 9. (CH, C-), 0. (CH, C-),. (CH, C-), 9.9 (CMe, C-7). 8 7 (E)--Acetylsulfanyl--hexen--ol (E)-b: H-NMR (0 MHz, [D ]acetone, 9 K): δ =.7 (dd, J =. Hz, J = 9.9 Hz, H, H-),.0 (dd, J =. Hz, J = 9. Hz, H, H-), (d, J = 9. Hz, H, H). C-NMR (90 MHz, [D ]acetone, 9 K): δ =. (CH, C- H 8), 0. (CH, C-),.7 (CH, C-).

16 ,-Bis(acetylsulfanyl)--hexanol c (mixture of diastereomers): H-NMR (0 MHz, [D ]acetone, 9 K): δ =..79 (m, H, H-),.9 (br s, H, H),..7 (m, 0 9 H H, H-). C-NMR (90 MHz, [D ]acetone, 9 K): δ = 9./9.7 (CH, C-), /.0 (CH, C-), 7./7.8 (CH, C-), 9.08/9.7 (CMe, C-7 or C-9).. Reaction of MVK (-buten--one) with thioacetic acid in [D ]acetone etup: Methyl vinyl ketone ( µl, 0. mmol), [D ]acetone (00 µl), CH Cl ( µl), thioacetic acid ( µl, 0. mmol). H K eq Ac H Ac d (Z)-d d Example spectrum: recorded 0 min after mixing ( H NMR, 0 MHz, [D ]acetone) Composition after 0 min: (8%), d (.%), (Z)-d (.%), d (%). elected spectral data of reaction mixture components: Methyl vinyl ketone (d): H-NMR (0 MHz, [D ]acetone, 9 K): δ =. (s, H, H-), H.9 (dd, J = 8.9, J =.8 Hz, H, H- ),..9 (m, H, H- and H-). C-NMR ( MHz, [D ]acetone, 9 K): δ =. (CH, C-), 9. (CH, C-), 8. (CH, C-), 98.7 (CMe, C-). H ' CH H

17 7 (Z)--Acetylsulfanyl--buten--ol (Z)-d. elected signals: H-NMR (0 MHz, [D ]acetone, 9 K): δ =.77 (d, J = 0.8 Hz, H, H-),.9 (s, H, Ac),. (d, J = 8. Hz, H, H-),. Ac (td, J = 8. Hz, J = 0.9 Hz, H, H-), 7. (s, H, H). C-NMR ( MHz, [D ]acetone, 9 K): δ =. (CH ), 9. (CH, C-),. (C, C-), 9. (C, C- ). H -Acetylsulfanyl--butanone (d): H-NMR (0 MHz, [D ]acetone, 9 K): δ =. (s, H, H-),.8 (s, H, H-),.77 (t, J =.8 Hz, H, H- or H-),.00 (t, J =.8 Hz, H, H- or H-). C-NMR ( MHz, [D ]acetone, 9 K): δ =. (CH, C-), 9.7 (C, C- or C-),. (CH, C-), 9. (CMe, C-).. Reaction of -penten--one with thioacetic acid in [D ]acetone etup: -penten--one ( µl, 0. mmol; purity ca 70%), [D ]acetone (ca. 00 µl), CH Cl ( µl), thioacetic acid (7 µl, 0. mmol). K eq H H e (Z)-e e Example spectrum: recorded 0 min after addition ( H NMR, 0 MHz, [D ]acetone) Composition after 0 min: (%), e (7%), (Z)-e (7.0%), e (9%); mesityl oxide (7%).

18 8 elected spectral data of reaction mixture components: -Penten--one (e): H-NMR (0 MHz, [D ]acetone, 9 K): δ =.88 (dd, J =.8 Hz, J =.7 Hz, H, H-),.7 (s, H, H-),.0 (dq, J =.9, J =.7 Hz, H, H-),.8 (dq, J =.9, J =.8 Hz, H, H-). C-NMR ( MHz, [D ]acetone, 9 K): δ = 8. (CH, C-),.9 (CH, C-),.8 (CH, C- ),.9 (CH, C-), 97.9 (CMe, C-). (Z)--Acetylsulfanyl--penten--ol (Z)-e: H-NMR (0 MHz, [D ]acetone, 9 K): δ =. (d, J =.9 Hz, H, H-),.7 (d, J = 0. Hz, H, H-),. (s, H, H-7),. (dd, J = H Hz, J = 0.9 Hz, H, H-),. (dq, J = 9.9 Hz, J =.8 Hz, H, H-), 7. (s, H, H). -Acetylsulfanyl--pentanone (e): H-NMR (0 MHz, [D ]acetone, 9 K): δ =.7 (d, J =.9 Hz, H, H-),. (s, H, H-),. (s, H, H-7),.7 (ddq, J = 7. Hz, J = 7. Hz, J = 0. Hz, H, H-),.8 (dd, J = 7. Hz, J =.8 Hz, H, H-),.8 (dqd, J = 7.,.9,.8 Hz, H, H-). C-NMR ( MHz, [D ]acetone, 9 K): δ =. (CH, C-), 0. (CH, C- or C- 7 7), 0. (CH, C- or C-7),. (CH, C-) 9.8 (CH, C-), 0.7 (CMe, C-).. Hemithioacetal from butanal and thioacetic acid.. Hemithioacetal from butanal and AcH in CDCl etup: Experiment performed by combining n-butanal (7 µl, 0.7 mmol), thioacetic acid (0 µl,. mmol) and CH Cl (8 µl) in CDCl (0 ml). H K eq H 7 elected NMR data of reaction mixture components: n-butanal (): H-NMR (0 MHz, CDCl, 9 K): δ = 0.97 (t, J = 7. Hz, H, H-),.7 (sext, J = 7. Hz, H, H-),. (t, J = 7. Hz, H, H-), (m, H, H-). C- NMR ( MHz, CDCl, 9 K): δ =.7 (CH, C-),.7 (CH, C-),.8 (CH, C-), 0.8 H (CH, C-). -Acetylsulfanyl--butanol (7): H-NMR (0 MHz, CDCl, 9 K): δ = 0.9 (t, J = 7. Hz, H, H-),.8 (sext, J = 7. Hz, H, H-),..9 (m, H, H-),. (s, H, H-),.. (m, H, H-). C-NMR (90 MHz, CDCl, 9 K): δ = 8.8 (CH, H C-), 0. (CH, C-),. (CH, C-), 7. (CH, C-), 78. (CH, C-), 00. (CMe, C-).

19 9 Example spectrum: recorded h after mixing ( H NMR, 0 MHz, CDCl ) Composition after h: (87%), (8.8%), 7 (90.%)... Hemithioacetal from butanal and AcH in C D etup: Experiment performed by combining n-butanal (7 µl, 0.7 mmol), thioacetic acid ( µl, 0.7 mmol) and CH Cl (8 µl) in C D (00 µl). H K eq H 7 Example spectrum: recorded 8 h after mixing ( H NMR, 0 MHz, C D ) (next page)

20 0 Composition after 8 h: ( 0%), (8.9%), 7 (80.0%). elected NMR data of components: n-butanal (): H-NMR (0 MHz, C D, 9 K): δ = 0. (t, J = 7. Hz, H, H-),.8 (sext, J = 7. Hz, H, H-),.7 (td, J = 7. Hz, J =.7 Hz, H, H-), 9.9 (t, J =.7 Hz, H, H-). C-NMR (90 MHz, C D, 9 K): δ =. (CH, C-),.7 (CH, C-),. (CH, H C-), 0. (CH, C-). -Acetylsulfanyl--butanol (7): H-NMR (0 MHz, C D, 9 K): δ = 0.79 (t, J = 7. H Hz, H, H-),.9.7 (m, H, H-),..87 (m, H, H-),.9 (s, H, H-),.7 (dd, J = 7. Hz, J =. Hz, H, H-). C-NMR (90 MHz, C D, 9 K): δ = 9. (CH, C-), 0. (CH, C-), 0.9 (CH, C-), 7. (CH, C-), 78.8 (CH, C-), 99. (CMe, C-).