The lithium compound (S)-7, formed by deprotonation of the (S)-S-1-phenylethyl thiocarbamate (S)-10, is configurationally stable at -70 degrees C. Even at elevated temperatures it racemizes only very slowly. It represents the first essentially enantiopure alpha-thiocarbanion derivative and can be utilized in asymmetric synthesis. Most electrophiles (except proton acids) add to (S)-7 with complete stereoinversion. Cleavage of the substitution products leads to practically enantiopure, tertiary 1-phenylalkanethiols.
Selbst bei 0°C ist das lithiierte Benzylthiocarbamat (S)‐1 konfigurationsstabil. Es wird von den meisten Elektrophilen unter Inversion der Konfiguration substituiert und ist das erste enantiomerenreine α‐Thiocarbanion, dessen Konfigurationsstabilität für präparative Zwecke ausreichend groß ist.
Only few synthesis have been reported for the 1-aza-4-oxa-7-thiobicyclo[3.3.0]octane system, such as for 1 [1] and 2 [2]. These heterocycles constitute an interesting type of amino acetal:
Abstract.A convenient method for the synthesis of the title compounds 4a,b, 3a,b via an intramolecular condensation of thiourethanes, derived from the acylation of enantioenriched α-thio benzyllithium compounds, is reported. The structure of one of the major diastereomers was elucidated by a singlecrystal X-ray analysis and compared to semiempirical calculations.1 ) X-ray crystal structure analysis During our studies on acylation reactions of the highly enantioenriched, configurationally stable S-lithiobenzyl thiocarbamate 6, [3] we unintentionally noticed a surprisingly facile method for the formation of the corresponding 8-oxo derivatives 3 and 4 (Scheme 1).The ketones (R)-7 are stereospecifically formed from the (S)-lithio derivative (S)-6 with complete stereoinversion on reaction with acid chlorides. When these were treated with small amounts of acidic catalysts under the usual conditions of deblocking, [3] the bicyclic compounds 3/4 were obtained instead of the expected N-(β-hydroxyalkyl)thiourethanes 8. Besides the major diastereomers 3a and 3b, small amounts of a second diastereomer (4a and 4b, respectively) were detected. The major diastereomer of the methyl-substituted derivative 3a formed suitable single crystals for an X-ray crystal structure analysis, which confirms its (5R,6R)-configuration [4] (Figure 1). C26 C27 C25 C24 C23 C22 C2 C21 S1 C8 O8 N7 C6 C61 C62 C31 C3 O4 C5 Fig. 1 X-ray crystal structure of 3a [4] Fig. 2 PM3-calculated structures for 3a (∆ f H = -47.33 kcal . mol -1 ) and 4a (∆ f H = -45.88 kcal . mol -1 ) O N S 1 H R R = 2-pyridyl O N S 2 H R = Me, Ph H R O N S O 3/4 R Me Ph R = Me, Et R Ph
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