Alcohols via xanthates

Olefin formation (preferentially less substituted) from alcohols via xanthate pyrolysis. 

Friedel-Crafts acylation 44, 956 halides from alcohols via xanthates 44,430... 

Chlorides from alcohols via xanthates Retention of configuration... 

FREUDENBERG - SCHdNBERG Xanthate Rearrangement Rearrangement of S-methyl xanthates to S-methyIdithiocarbonatas (conversion of alcohols lo thiols via xanthates, also phenols to thiophenols via thiocarbamates). 

Alcohols can be dehydrated via xanthate esters at temperatures that are much lower than those required for acetate pyrolysis. The preparation of xanthate esters involves reaction of the alkoxide with carbon disulfide. The resulting salt is alkylated with methyl iodide. 

BARTON MC COMBIE Deoxygenation Deoxygenation ot secondary alcohols to hydrocarbons (via xanthates)... 

Defunctionalization. Hydrosilanes are better reagents than hydrostannanes for deoxygenation of alcohols via their xanthates. Thus, reasonably good results are obtained by using the symmetrical tetraphenyldisilane under free radical conditions. More... 

It is a useful reagent for deoxygenating phosphine oxides to phosphines, and will redueed chiral phosphine oxides to chiral phosphines with retention of eonfiguration [Marsi J Org Chem 39 265 1974]. It is a reducing agent in radical reactions [Perez et al. J Org Chem 52 5570 1967], and deoxygenates primary and seeondaty alcohols via their xanthate or thionocarbonate esters (prepared from aryl-OCSCl and an inhibitor, e.g benzoyl peroxide) in 60-100 minutes in >87% yields [Barton et al. Synlett 435 7997]. [Beilstein 16 III 1198,16 IV 1360.]... 

Alcohols can be dehydrated via xanthate esters at temperatures much lower... 

Deoxygenation of Alcohols. A new application of DTBP is its use as an initiator for radical-mediated deoxygenation of alcohols via the corresponding xanthate using diphenylphosphinic acid as the hydrogen source (eq 27). ... 

Bernard, J., Favier, A., Zhang, L. et al. (2005) poly(vinyl ester) star polymers via xanthate-mediated living radical polymerization from poly(vinyl alcohol) to glycopolymer stars. Macromolecules, 38,5475-5484. 

Carbonates and carbamates are reported to be intermediate in stability and easier to prepare than xanthates. " They can conveniently be prepared directly from the alcohol in high yield and give clean conversion to olefins. Cholesta-3,5-diene, for example, can be readily obtained via the phenylcar-bamate (114) or ethylcarbonate (115) of cholesterol. Such esters appear to have been somewhat neglected as synthetic intermediates. 

The thermolysis of xanthates derived from primary alcohols yields one olefin only. With xanthates from secondary alcohols (acyclic or alicyclic) regioisomeric products as well as fi/Z-isomers may be obtained see below. While acyclic substrates may give rise to a mixture of olefins, the formation of products from alicyclic substrates often is determined by the stereochemical requirements the /3-hydrogen and the xanthate moiety must be syn to each other in order to eliminate via a cyclic transition state. 

The keto group of the trisubstituted dioxanone 133 generated by ozonolysis was removed by radical deoxygenation according to the Barton-McCombie protocol [80] via the alcohols 134 and the corresponding xanthate, leading after deben-zylation to the dioxane 135 in excellent yield. After conversion to the tosylate, cleavage of the acetonide and protection of the secondary alcohol function as a TBS ether provided access to oxirane 128 by cyclization with NaH in 99% yield and in virtually diastereo- and enantiomerically pure form (de, ee > 96%). 

Not surprisingly, if the ketone in the starting material is flanked by an unsaturated functional group i.e. C02Me or NO ), dehydration gives exclusively the a,P-unsaturated product (the p,y-unsaturated regioisomer is available via other methods, discussed below). While the methods described above involve direct dehydration of the alcohol, a number of reduction-dehydration sequences have been conducted by initial reduction of the ketone and subsequent derivatization of the resultant alcohol (as the phenyl methanesulfonate, methyl p-toluenesulfonate, xanthate, acetate, methyl ether, etc.). This type of reduction-dehydration sequence is discussed in detail in Volume 6, Chapter 5.1. 

Thus secondary or tertiary alcohols ROH can be reduced to RH via their thiocarboxy-lates R C(S)OR, or dithiocarbonates [xanthates, R S(CS)OR the Barton McCombie reaction] 147-149 for example cholesteryl thiobenzoate or S-methyl dithiocarbonate gives cholest-5-ene in 90% and 78% yield respectively (equation 15-47, R = Ph or MeS). The reaction works best for secondary alcohols the 0-t-alkyl xanthates are often rather unstable at room temperature, though the reactions can be carried out at low temperature with initiation by Et3B/02. The primary alkyl xanthates undergo C-O fission only at higher temperatures, when other reactions may compete. The thiocarbonyl imidazolides provide a variant on this theme, and the cholesterol derivative (15-5, R = /V-imidazolc) reacts with tributyltin hydride to give an 18% yield of cholestene.147... 

Chugaev reaction. Formation of olefins from alcohols without rearrangement through pyrolysis of the corresponding xanthates via cis elimination. 

This reaction has been modified to generate xanthates in one step via the reaction of alcohols with CS2 and NaOH (or KOH)7... 

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