Dithioacetal 426 Subject

Among the facts supporting this mechanism (which is an A-Se2 mechanism because the substrate is protonated in the rate-determining step) are (1) lsO labeling shows that in ROCH=CH2 it is the vinyl-oxygen bond and not the RO bond that cleaves 497 (2) the reaction is subject to general acid catalysis 498 (3) there is a solvent isotope effect when D2O is used.498 Enamines are also hydrolyzed by acids (see 6-2) the mechanism is similar. Ketene dithioacetals R2C=C(SR )2 also hydrolyze by a similar mechanism, except that the initial protonation step is partially reversible.499 Furans represent a special case of enol ethers that are cleaved by acid to give 1,4 diones. Thus... 

Fig (19) Octalin ketal (163) is converted to kete dithioacetal (164) by the cleavage of ketal function and condensation with carbon disulfide and methyl iodide. Subjection of (164) to the action of dimethylsulfonium niethylide and acid hydrolysis leads to the formation of unsaturated lactone (165).lts furan silyl ether derivative is caused to undergo Diets-Atder reaction with methyl acrylate to obtain salicyctic ester (166) which is converted by standard organic reactions toabietane ether (167). It is converted to aiiylic alcohol (168) by epoxidation and elimination. Alcohol (169) obtained from (168) yields orthoamide which undergoes transformation to amide (170). Its conversion to the previously reported intermediate has been achieved by epoxidation, elimination and hydrolysis. 

The C23-C26 segment 173 was prepared from 180, which was derived from ribose (O Scheme 20). After 0-benzylation of 180, the resulting benzyl ether was treated with MeMgCl and CuBr Me2S to afford 181. Thiol acetal formation followed by selective silyla-tion provided 182. The dithioacetal was cleaved and the resulting aldehyde was reduced with NaBH4 to afford a diol, which was subjected to direct epoxidation to provide 173. 

The course of reactions with mercury(II) chloride is subject to control by experimental conditions, and by substitutive and stereochemical constraints within the particular dithioacetal undergoing hydrolysis. Much of the synthetic versatility of dithioacetals derives from this control, and various aspects have been reviewed in detail in this Series6-8 and elsewhere.261... 

Reaction of 9 with paraformaldehyde afforded the 1,3-dioxane 10, which was subjected to oxidative hydrolysis of dithioacetal group with mercuric perchlorate to form 11, the AB ring moiety of sesbanimides 1 and 3, in 35% overall yield from 4. 

Martin and Zinke have given a full account of their synthesis of KDO in which the molecule is assembled around a furan template (see Vol. 23, p.162-3).39 The dithioacetal (35) of KDO (see also Vol. 23, p.161-2) can be subjected to iodonium-ion induced cyclisation to give access to, depending on the conditions, the thioglycoside (36) or the alkene (37) (36) can be used to make p-glycosides of KDO, whilst (37), when activated with phenylselenenyl triflate, acts as a precursor to a-glycosides. f Other references to glycosides... 

O-Alkyl a-(trimethylsilyl)thioacetates and S-alkyl a-(trimethylsilyl)dithioacetates 119 can be deprotonated and subjected to Peterson reactions with aldehydes to give the a,jS-unsaturated thiocarboxylic acid O-esters and dithiocarboxylic acid S-esters 120, respectively (Scheme 2.72). The alkenes are obtained -selectively [200]. 

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