Oxazolidine-2-thione ring

Selective oxazolidine-2-thione ring S-alkylation and N-acylation s. 18, 680... 

These examples and those in Scheme 2.6 illustrate the key variables that determine the stereochemical outcome of aldol addition reactions using chiral auxiliaries. The first element that has to be taken into account is the configuration of the ring system that is used to establish steric differentiation. Then the nature of the TS, whether it is acyclic, cyclic, or chelated must be considered. Generally for boron enolates, reaction proceeds through a cyclic but nonchelated TS. With boron enolates, excess Lewis acid can favor an acyclic TS by coordination with the carbonyl electrophile. Titanium enolates appear to be somewhat variable but can be shifted to chelated TSs by use of excess reagent and by auxiliaries such as oxazolidine-2-thiones that enhance the tendency to chelation. Ultimately, all of the factors play a role in determining which TS is favored. 

In carbohydrate chemistry, the most described method for the preparation of saccharidic thionocarbamates involves preliminary introduction of the amine function on a partially or non-protected saccharidic template. The condensation of amino sugars with carbon disulfide or thiophosgene leads to cyclization in 1,3-oxazolidine- or l,3-oxazine-2-thiones. This reaction involves the formation of an intermediate isothiocyanate, which reacts further with a 3- or y-located hydroxyl group. The viability and facility of this process depends on the saccharidic ring size and the inherent strain. Some major rules can be put into light from the cases studied 30... 

To increase the yields of the ring closure reactions, a new method was developed that was successfully applied for the synthesis of alicyclic fused systems of both the parent oxazolidine-2-thione and tetrahydro-1,3-oxazine-2-thione (85S1149). As an example, the synthesis of 2-thioxoperhydro-l,3-benzoxazine 103 is described. The dithiocarbamate 101, prepared from the amino alcohol 100, carbon disulfide and triethylamine, was treated with ethyl chloroformate in the presence of triethylamine, to give the thioxo derivative 103 via the transition state 102 (85S1149). In this way, the fused-skeleton thioxooxazines (91, X = S, 92) can be prepared with considerably higher yields (50-70%) than by the earlier methods (85S1149). 

As in other applications of 7V-acyl-l,3-oxazolidin-2-ones, 2-thiones, and sthiazolidine-2-thiones, chelation of the Lewis acid center for restricted rotation is considered decisive for the reactions occurring under the influence of the Ti-TADDOLates. Generally, the attack of the nucleophilic component (diene or ene) on the chelated electrophile occurs from the bottom face if the chelate ring is drawn as shown in structures (17) and (18), (19), and (lO). For the oxazolidinones, this means that the trigonal a-carbonyl center is approached from the (/ e)-face when an (/ ,/ )-Ti-TADDOLate is used (rel. topicity like) the mechanism of this reaction has been discussed. ... 

The same group prepared N-(/S-D-glucopyranosyl)oxazolidine-2,4-diones [146] and N-(/l-D-glucopyranosyl)imidazolidine-2,4-diones [147] by desulfurization-condensation of glucosyl isothiocyanate with a-hydroxyacids and J -substituted a-aminoacids, respectively, in the presence of silver triflu-oroacetate and triethylamine. 4-Glucosyl-l,2,4-oxadiazolidine-5-thiones were also prepared by reaction of protected glucosyl isothiocyanates with an ox-aziridine ring [148]. 

A large number of azomethine nitrofurans containing various heterocyclic rings have been synthesized the most impKjrtant of these contain either a hydantoln Table 6.5) or oxazolidinone Table 6.6) ring. Nitrofurans containing the following rings have also been studied 3-oxazolidin-2-one XI) >, 3-rhodanine XII), 3-imidazolid-2-one XIII), 1-hydantoin l-imidazolidine-2-thione 4-1,2,4-triazole (ZF/) , 1-... 

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