Dithioacetals chain extension

Methods of chain extensions generally use linear sugars with free aldehydes mostly prepared via dithioacetal formation, protection of the remaining hydroxyl groups and aldehyde recovery. This allows one to use all methods applicable to aldehydes. Among others, the condensation of ethyl diazoacetate has been studied in detail by L6pez-Herrera [207] in a new synthesis of KDO by a two carbon homologation of an open-chain mannose derivative. 

Ketene dithioacetals should also be mentioned these are readily prepared by dehydrogenation of saturated dithioacetals, or dihydrohalogenation of 2-dichloromethyl-1,3-dithiane followed by chain extension (Scheme 4). ... 

The isopropylidenation of aldose trimethylene dithioacetals is covered in Chapter 6 and the acetolysis of aldose diethyl dithioacetals in Chapter 7. The C-C-l(C-2 bond cleavage of the a-oxoketene dithioacetal 7 on exposure to mineral acid is referred to in Chapters 13 and 14. The phenylthio precursor 8 of 2-keto-3-deoxy-D-flraZ //io-heptonic acid has been prepared by chain-extension of 2,3 4,5-di-O-cyclohexylidene-D-arabinose with PhSCH2C02Me, followed by de-protection. ... 

The pentoses may be converted by the conventional sequence (5) of dithioacetal formation, acetylation, and demercaptalation into the respective aldehydo-oentose 2,3,4,5-tetraacetates, which afford by Wittig chain-extension (2) the desired dienophiles. Thus aldehydo-D-arabinose 2,3,4,5-tetraacetate reacted with Ph3PCHC02Me in boiling benzene to give 90% of the pure, crystalline E-unsaturated sugar derivative 6, and the corresponding enantiomer was likewise prepared from L-arabinose. 

Ti complexes have been extensively used for the stereoselective introduction of side chains in steroidal molecules, e.g. pregnenolone acetate and the steroidal C-22 aldehyde (73 equation 29). In these cases satisfactory results are only obtained by using Ti reagents, like methyl, ds-methyl and allyl derivatives, due to the strong steric hindrance that affects the position to be attacked. 1,3-Anti dia-stereoselecdon in the addition of alkyltitanium reagents to chirally -substituted aldehydes having a dithioacetal group at the a-position (75) was also observed (equation 30). ... 

---