. J. Chem. 58,604 (1980).Variation of the conformational properties in a series of 4-R-thiazolidine-2-thiones (4-R = Me, Et, iPr, tBu, OH) and 5-Rthiazolidine-2-thiones (5-R = Me, Et, iPr, tBu) is deduced from 'H nmr (Karplus 'J, chemical shifts, 'J,,,, 'JS5,) 13C nmr data and CNDO-2 calculations. In the absence of any transannular interaction, tricinal constraints and van der Waals requirements of the substituents control the conformational equilibrium. The pseudo-axial arrangement is favoured when 4-R = Me, while the substituent presents no preferential orientation when 5-R = Me. As the size of the 5-alkyl and the 4-alkyl substituent increases, its pseudo-equatorial orientation and the predominance of the C4C5 out-of-plane half-chair conformation are favoured. This does not lead to conformational exclusivity even when 4-R or 5-R is tBu. When 4-R is hydroxy, there is a definite pseudo-axial conformation (anomeric effect) and the ring preference is of the C4 out-of-plane type.F. CHANON, M. RAJZMANN, M. CHANON, J. METZGER, C . POUZARD et T. DRAKENBERG. Can. J. Chem. 58.604(1980). Les Cvolutions conformationnelles dans les R-4 thiazolidinethiones-2 (R-4 = Me, Et, iPr, tBu, OH) et les R-5 thiazolidinethiones-2 (R-5 = Me, El, iPr, tBu) sont deduites de l'ensemble des rksultats rmn 'H ('J- Karplus,6,, 'J,,,, 2J44,), rmn "C et calculs CNDO-2. En I'absence d'interactions transannulaires, les contraintes cicinales et de van der Waals des substituants contr6lent i'equilibre conformationnel. Lorsque R-4 = Me, sa prdfkrence est Iigerement pseudo-axiale tandis que le substituant R-5 = Me n'a pas d'orientation spicifique. On observe, simultanement a une augmentation de la taille du substituant alkyle, une orientation de plus en plus equatoriale de ce dernier et une predominance accrue de la conformation demi-chaise avec C4C5 hors plan. Ceci ne conduit pas toutefois a un blocage conformationnel mCme pour le tBu. Pour R-4 = OH, une nette preference pseudo-axiale se manifeste (effet d'anomirie) et le plissement du cycle est du type enveloppe avec C4 hors du plan du thiocarbonyle.
IntrodustionHaving introduced substitution at C4 (Pb, Ic, Id, In the preceding paper (1) we have shown for l e 7 I f ) and C 5 (2b, 2c, 2 d j 2e)9 i.e. thiazolidine-2-thione ( l a ) the existence of equilibrium Tal :The conformer stability increases from left to right and the C4C5 torsional angle was evaluated to be ca. (Table 2) and 'H nmr experi-ppm) and the 4-R-THTH-2 series (Aa4,, = + 1.12 mental data (Table I) Table 3. They reveal a slight pseudo-equatorial preference exhibited by the 4-methyl derivative 16 and a slight pseudo-axial one for the 5-methyl derivative 26. The orientation of R is increasingly equatorial with increasing substituent size, particularly so for 4-R or 5-R = Et and iPr. A pseudo-equatorial preference close to 100x is not observed. The effective orientation of the methyi substituent is supported by substituent effects on 13G chemical shifts due to the introduction of methyl group (Table 4). a-and P-effects are both deshield...