Pergamon Tetrahedron: Asymmetry, Vol. 7, No. 9, pp. 2603-2606, 1996 Copyright © 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0957-4166/96 $15.00 + 0.00 PII: S0957-4166(96)00334-5 Two New Chiral Equivalents of H2S: a Thio- and a Dithiocarboxylic Acid Pedro de March,* Marta Figueredo,* Josep Font, Llu'isa Gonzfilez, and Antonio Salgado Departmnent de Qufmica. Universitat Autbnoma de Barcelona. 08193 Bellaterra (Barcelona). Spain. Abstract.- The syntheses of the two new chiral mercapto derivatives (R)-2-[No(1-phenylethyl)- amino]- 1-cyclopentenedithiocarboxylic acid, 5, and (1S,4R)- 1 -(4,7,7-trimethyl-3-oxo-2- oxabicyclo[2.2.1]heptane)thiocarboxylic acid, 9, are described through easy transformations in good yields.Copyright © 1996 Elsevier Science Ltd The preparation of non-racemic chiral molecules becomes every day a more important and necessary goal in organic chemistry. During some of our synthetic studies we were faced with the problem of generating a mercaptocarboxylic acid derivative with a stereogenic center at the 13-position. The conjugate addition of heteronucleophiles to ct,13-unsaturated carbonyl groups is one of the most general methods for the [3- functionalization of carbonyl derivatives, but the number of described examples of asymmetric Michael-type reactions using sulfur nucleophiles is relatively small, l To the best of our knowledge, there is only one described case in which an enantiopure thiollg is used as nucleophile in a conjugate addition and the thus formed adducts are subjected to diastereoisomeric separation followed by an elimination of the sulfur derivative, but they are not used as synthetic intermediates incorporating the sulfur atom. Probably, the comparatively small number of examples of asymmetric conjugated addition using sulfur nucleophiles is due to the fact that the synthesis of non-racemic chiral thiols has not received much attention. 23 To solve our synthetic problem we visualized the use of chiral mono- or dithiocarboxylic acids as the most appropriate derivatives, since hydrolysis of the corresponding adducts would generate the free thiol function. The number of known enantiopure thiocarboxylic acids is very small. 4 Herein we report the easy preparation of two new sulfur derivatives, which are chiral synthetic equivalents of hydrogen sulfide. 2 Our first target was the 13-aminodithioic acid 5, whose synthesis may be undertaken by two approaches according to general methods for the preparation of dithioacids previously described (Scheme 1): condensation of cyclopentanone, 1, with a chiral amine, for instance 2, and subsequent reaction of the formed ketimine 3 with carbon disulfide 5 or by an amine exchange reaction from the enaminodithioacid 4. 6 We tried the first route and the imine 3 was obtained as a colorless liquid after distillation in 95% yield. We have fully characterized the known compound 37 by its spectroscopic data, specific rotation at the sodium D line, and elemental analysis. Unfortunately, none of our attempted reactions between 3 and carbon disulfide led to the isolation of the desired thioacid 5, but instead the salt 68 was obtained. The second route proved more successful and the reaction of the dithiocarboxylic acid 49 with the amine 2 gave the desired new compound 5 as a yellow oil in 53% yield. The I3C nmr spectrum of compound 5 shows absorptions at 5 190.1, 170.5, and 118.8 corresponding to the thiocarboxylic group and the [3 and ct olefinic carbon atoms respectively. The uv spectrum of 5 presents a maximum at ~. = 416 nm with e = 30500 and the specific rotation [aiD 20 of 5 has a value of 2603