Dithiocarbonates deoxygenation

Quebrachitol was converted into iL-c/j/roinositol (105). Exhaustive O-isopropylidenation of 105 with 2,2-dimethoxypropane, selective removal of the 3,4-0-protective group, and preferential 3-0-benzylation gave compound 106. Oxidation of 106 with dimethyl sulfoxide-oxalyl chloride provided the inosose 107. Wittig reaction of 107 with methyl(triphenyl)phos-phonium bromide and butyllithium, and subsequent hydroboration and oxidation furnished compound 108. A series of reactions, namely, protection of the primary hydroxyl group, 0-debenzylation, formation of A-methyl dithiocarbonate, deoxygenation with tributyltin hydride, and removal of the protective groups, converted 108 into 7. 

D. Crich, On the use of S-(4-alkenyI)-dithiocarbonates as mechanistic probes in the Barton-McCombie radical deoxygenation reaction, Tetrahedron Lett. 29 5805 (1988). 

Deoxygenation of alcohols.1 Dithiocarbonates are converted to the corresponding alkane on treatment with Bu3SnH (1 equiv.) and (QHS),B (1 equiv.) in benzene. 

Table 5.1 Deoxygenation of O-cyclododecyl S-methyl dithiocarbonate with tetraalkylammonium hypophosphites in water.

In this reaction, tributylstannane could undoubtedly be replaced by hypophosphorous acid (HjPO,) or a triethylammonium salt, as in the case of the deoxygenation reaction described at the end of this chapter. The most direct method to deoxygenate an isolated hydroxyl group is by the formation of dithiocarbonate (reaction 5.20). 

Further studies of the deoxygenation of alcohols by triplet sensitisation and SET to their S-methyl dithiocarbonates under a variety of conditions have been reported. The properties of 2,4,6-triphenylthiapyrilium tetrafluoroborate as an SET sensitiser have been discussed. The triplet excited state of 6H-purine-6-thione acts as an electron donor for p-dinitrobenzene and as an electron acceptor for tetramethylbenzidine. Photolysis of sulfuryl chloride in the presence of tetramethylsilane yields exclusively Me3SiCH2S02Cl. In the presence of added yttrium(III) chloride or sulfur, the reaction is less selective and Me3SiCH2Cl is also obtained. In contrast the presence of these photocatalysts in the photoreaction of sulfuryl chloride with hexamethyldisiloxane yields solely Me3SiOSi-Me2CH2S02Cl, whereas in their absence Me3SiOSiMe2CH2Cl is also obtained. ... 

Lanosteryl S-methyl dithiocarbonate in xylene added during 0.5 hr. under argon to a stirred and refluxing mixture of tributylstannane and xylene, and refluxing continued 2 hrs. -> lanosta-8,24-diene. Y 83%. - This reduction is part of a method for the deoxygenation of sec. alcohols. F. e. and related reactions s. D. H. R. Barton and S. W. McCombie, Soc. Perkin I 1975, 1574. 

Additionally, the reduction process of Barton and McCombie has been developed into a useful technology. The Barton-McCombie procedure (Scheme 8.22) involves the conversion of an alcohol to a thiocarbonate (or dithiocarbonate) derivative (usually in two steps) and the reaction of the latter with tributyltin hydride and azobisisobutyronitrile (AIBN) (bis(l-cyano-l-methylethyl)diazine, [(CH3)2C(CN)N=NC(CN)C(CH3)2]). As shown in Scheme 8.22, decomposition of AIBN produces a radical, which, in turn, abstracts hydrogen from tributyltin hydride, generating the tributyltin radical. The reaction of the latter with sulfur and the subsequent decomposition of that intermediate produces a deoxygenated. 

In addition to carbonyl reductions, PMHS can also be used in deoxygenations. For example, PMHS, Nal, and TMSCl will generate benzyl iodides from benzaldehydes (eq 16). Unfortunately this solventless transformation does not work well with ketones or aliphatic aldehydes. Fu and co-workers have shown that PMHS is instrumental in performing Barton-McCombie deoxygenations of thionocarbonates with only catalytic amounts of (Bu3Sn)20 (eq 17). Interestingly, dithiocarbonates do not respond uniformly well to similar conditions. ... 

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