Subject thiocarbonyls

The preparation and reactions of V-thiocarbonyl carbohydrate derivatives, such as sugar isothiocyanates, thioamides, thioureas, thiocarbamates and their conjugates have been the subject of valuable comprehensive as well as specialized accounts, which should be consulted for details.198... 

A relevant reductive process, which has found wide application in organic synthesis, is the deoxygenation of alcohols introduced in 1975 by Barton and McCombie [58]. Reaction (4.28) shows that the thiocarbonyl derivatives, easily obtained from the corresponding alcohol, can be reduced in the presence of BusSnH under free radical conditions. The reactivity of xanthates and thiocarbonyl imidazolides [58] was successfully extended to 0-arylthiocarbonates [59] and (9-thioxocarbamates [60]. Several reviews have appeared on this subject, thus providing an exhaustive view of this methodology and its application in natural product synthesis [61-64]. 

Recrystallization of thioamides 24f-h gave colorless or slightly yellow prismatic crystals [49]. All three thioamides were subjected to X-ray single crystal analysis, and it was revealed that they prefer the same conformation 24-1 in which a benzyl group was placed close to the thiocarbonyl group, the same as the major isomer in solution. The alkenyl double bonds are perpendicular to the thioamide group, and twist in the range of 87.9° to 100.5°. 

Historically, the first reactions involving thiocarbonyl ylides involve the preparation of thiiranes and 1,3-dithiolanes from diazomethane and thiocarbonyl compounds reported early in the last century by Staudinger and co-workers (12,13). Similar reactions have been smdied by Schonberg and co-workers (14—16) during the 1960s, but neither was the reaction mechanism understood nor have thiocarbonyl ylides been recognized as key intermediates. [For some remarks to this subject see (8) and (10) in (17).]... 

Whereas the subject of basicity of C=0 groups has already been covered by Palm and coworkers1, in this chapter we summarize more recent results and significant developments in the field of nitrones, nitriles and thiocarbonyls. This chapter is not exhaustive in scope, but rather consists of surveys of the most recent two decades of work in this still developing area. As indicated by the title of this contribution, we emphasize the more physical aspects such as acidity, basicity and proton affinity less attention is paid to synthesis, structure and bond theory which can be found in other specialized chapters of this book. 

In Section III.K we have briefly summarized the major advances in the preparation of thiocarbonyl compounds stabilized by organometallics. The synthetic applications of this kind of reagents have experienced impressive growth in recent years and major contributions will be collected in this section, although the interested reader is referred to specialized Serials on this subject. 

Thiocarbonyl oxides are a subject of active investigation. The natural occurrence of sulfines and related compounds in plants of the genus Allium (onion, garlic, etc.) is included in a superb and extensive review by Block [91]. Two detailed papers [92, 93] report the isolation of zwiebelanes from onions and their chemical synthesis involving intermediate sulfines produced by oxidation of di-l-propenyl disulfide, subsequent sulfoxide accelerated [3.3] sigmatropic shift and the [2+2] cycloaddition of the C=S and C=S=0 moieties. A further article [94] provides a great deal of information on the mechanism of formation of (Z)-propanethial S-oxide, the lachrymatory factor of the onion, as well as its chemical synthesis and reactions. Techniques of analysis of the volatiles of onions have been further improved [95]. 

When chiral thiocarbonyl S-oxides derived from (S)-proline, prepared from the appropriate l-(trimethylsilylalkylsulfonyl)-2-(alkoxymethyl)pyrrolidine, are subjected to Diels-Alder reaction with 2,3-dimcthyl-l,3-butadiene, the 2-substituted 2-[(2-alkoxymethyl-l-pyrrolidinyl)sul-fonyl]-3,6-dihydro-4.5-dimethyl-2/f-thiopyran 1-oxides 10A and 10B are formed as a mixture of diastereomers. Their ratio depends on the nature of the substituents R1 and R2 as well as on the reaction temperature. The diastereomeric excess values, determined by H NMR in the presence of Yb(tfc)3 or by HPLC, vary from 0 to 41 % 84. 

The photochemistry of organosulfur compounds in general has been the subject both of annual reports [1,2] and of timely scattered reviews [3-5], Light-induced reactions of specific chromophores, such as sulfoxides and sulfones [6], thiocarbonyl compounds [7], and thiols [8] have been discussed in more detail, as has the photochemistry of thiophenes and certain other specific ring systems [9,10],... 

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