2- Acyl-l,3-dithianes

Besides 1,3-oxathianes, the 1,3-dithiane 1-oxide moiety can be used for directing the nucleophilic addition of an organometallic reagent to a carbonyl group in a diastereoselective manner. The addition of methylmagnesium iodide to the 2-acyl-l,3-dithiane 1-oxide 23A leads exclusively to the diastereomer which is formed by Re-side attack. On the other hand, addition... 

An unexpected reactivity in the functionalization of 2-acyl-l,3-dithianes has been reported by Mioskowski and co-workers. They found that 2-acyl-l,3-dithianes with no further heteroatom at the acyl side chain react with aldehydes to give 2-acyl-2-hydroxyalkyl-l,3-dithianes, whereas a silyl-protected hydroxy group in the side chain of the 2-acyl-l,3-dithiane led to formation of the aldol product at the opposite site of the carbonyl group. Acyl chlorides always react with 2-acyl-l,3-dithianes to give the enol esters (Scheme 81) <2003TL213>. 

The chemistry of chiral 1,3-dithiane 1-oxides, in particular their use as chiral auxiliaries, has been reviewed <19980PP145>. Some further developments in this field are the stereoselective a-alkylation with alkyl halides <1997T13149> or a-hydrazination with di-fert-butyl azodicarboxylate (DBAD) <2000T9683>. The carbonyl group of 2-acyl-l,3-dithiane 1-oxides was also used as an electrophile (Scheme 82). Interestingly, acyclic enolates react with these substrates to give a 95 5 mixture of anti- and ry -adduct, whereas cyclic enolates produce a mixture of anti- and ry -adduct in 8 92 ratio <2000JOC6027>. 

Page et al. (see [298] and references therein) have shown that generally excellent stereocontrol in organic reactions can be obtained by using DITOX (1,3-dithiane-l-oxide) derivatives as chiral auxiliaries. The one-pot stereo-controlled cycloalkanone synthesis given here outlines some aspects of the chemistry worked out for efficient acylation-alkylations steps. Of note are the use of N-acyl imidazoles under mixed base (sodium hexamethyldisilazide/n-butyllithium) conditions to yield the lithium enolates of 2-acyl-l,3-dithiane-l-oxides) and the sequential alkylation-cyclization of the latter (steps (iv) and (v)). 

Oxidative hydrolysis of 2-aeyl-l -dithiane derivatives. A study of the hydrolysis of 2-acyl-1,3-dithianc derivatives by mercuric chloride (HgCl ) has been published. However, hydrolysis of 2-acyl-l,3-dithianes is slow. In this case use of N-haiosuccini-mide reagents is recommended. Either N-bromo uccinimide or N-chlorosuccinimidc-silver nitrate is suitable for oxidative hydrolysis of 1,3-dithiane derivatives. The N-halosuccinimide reagents are useful for hydrolysis of 2-acyl-1,3-dithianes to 1,2-dicarbonyl compounds. 

Of related interest is the diastereoselective addition of Grignard reagents to 2-acyl-l,3-dithiane 5-oxides [75], which gave results in accordance with chelated Cram-type transition states [76] involving equatorial sulfoxides (Scheme 3.46). 

Over the last two decades, 1,3-dithianes have become ubiquitous as acyl anion equivalents in organic synthesis, with thioacetal chemistry having already been well established [107]. It is therefore somewhat surprising to find that 2-acyl-l,3-dithianes, which may easily be prepared in excellent yield from dithiane 2-anions, have received relatively little attention [108]. The development of 2-acyl-l,3-... 

Reactions of organocuprates with a,P-unsaturated 2-acyl-l,3-dithiane 1-oxides have also been studied (Scheme 4.61) [114]. 

Oxidative hydrolysis of 2-alkyl- and 2-acyl-l,3-dithians by iV-halogeno-succinimides is recommended as a high-yield method of carbonyl regeneration the combination of iV-chlorosuccinimide and silver(i) ion is suitable for reaction with unsaturated 1,3-dithians. In a related oxidative procedure, ketones and aldehydes can be readily regenerated from their ethylene thio- or hemithio-acetals by treatment with aqueous chloramine-T. 

On the other hand, the reactions of 2-lithio-2-trimethylsilyl-l,3-dithiane with esters and thioesters give not the ketene dithioacetals, but the 2-acyl-l,3-dithianes 81 (Scheme 2.50). The possibly formed silyl enol ethers 80 undergo subsequent hydrolysis to give 81 [141],... 

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