Methylxanthate

The decomposition of the S-methylxanthate ester of 1,1-dimethyl-1-silacyclopcnt-4-cn-3-ol takes place in refluxing ether, but this ester cannot give the silole 4 which dimerizes in situ (61% yield), together with siloxane formation (Me2SiC4H5)20 (31%) due to a ft C—Si elimination14. 

S -Methylxanthates 62 derived from the same alcohols 50 are thermally less stable than the corresponding carbamates 57. They decompose during their synthesis according to two competitive elimination pathways, ft C—H and ft C—Si (3/2) eliminations14 (Scheme 12), giving rise to a lower metallole yield than from carbamates 57. 

The structure of the methylxanthate analogue, (Ph3P)2Cu(S2COMe), is also known and this essentially adopts the same structure as described above (127). The remaining copper xanthate structures are mixed-metal species. 

The efficient reductions observed for various organic halides when treated with Et3SiHin two-fold excess in the presence of catalytic amounts of a thiol and di-tert-butyl hyponitrite as initiator confirmed the validity of this approach. Silane-thiol couples were also used, in boiling octane with di-terf-butyl peroxide as initiator, for the reduction of the methylxanthate of 1,2 5,6-di- O -isopropylidene-a-D-glucofuranose, to yield the corresponding deoxy sugar in 60%173 and 70%174... 

Ring opening of 5,6-dideoxy-5,6-epithio-l,2-0-isopropylidene-/3-L-idofiiranose with potassium methylxanthate leads to 1,2-O-isopropyl-idene-5,6-dithio-5,6-S-thiocarbonyl-/3-L-idofuranose, which can be reductively cleaved with lithium aluminum hydride to 1,2-O-isopro-pylidene-5,6-dithio-)3-L-idofuranose. Acid hydrolysis of the latter compound would be expected to give a 1,6-anhydroidose derivative having a sulfur atom in both the pyranose ring and the anhydro ring, but a definitive product was not isolable. 

Unsaturated derivatives of cellulose containing double bonds directly in the pyranose ring have b n synthesized by the thermal decomposition of cellulose methylxanthate or bis-xanthate (39,40), which proceeds as follows ... 

Synthesis of olefins from stericaUy hindered alcohols. Gerlach et al. have reported a successful variation of the Chugaev reaction applicable to the preparation of temperature-sensitive olefins from sterically hindered alcohols. Thus /ran.v./ran.v-spiro[4.4]nonane-1,6-diol (2) is converted into the thiocarbonate O-ester (3) by reaction with (1) in pyridine for 4 hr. at room temperature (80% yield). This derivative decomposes smoothly into the diene, 1,6-spiro[4.4]nonadiene (4), at a temperature of > 135°. In this case pyrolysis of the diacetate or of the di-S-methylxanthate failed to give the diene. 

No examples of gas-phase rearrangements of esters have so far been reported. However, Bunton and co-workers (1961) have examined bornyl and isobornyl methylxanthates and benzoates in the liquid phase... 

Alternative routes to compounds of this class involve the treatment of 5,6-anhydrides with methyllithium, and the application of the Chugaev reaction. Thus, 3-0-benzyl-5,6-dideoxy-l,2-0-isopropylidene-a-n-a 2/Zo-hex-5-enose (113, R = CHjPh) has been obtained from both 5,6-anhydro-3-0-benzyl-l,2-0-isopropylidene-a-D-glucofuranose and the related -L-idose derivative, and has been identified with the product prepared from 3-0-benzyl- 1,2-0-isopropylidene-5,6-di-0-p-tolysulfonyl-a-D-glucofuranose. Methyl 5,6-dideoxy-2,3-0-isopropylidene-/3-D-n6o-hex-5-enofuranoside (114) was obtained from the corresponding n-allose 5-methylxanthate derivative. 

From the [a]D and ORD of these alkaloids it was concluded that they all belonged in absolute stereochemical series of (—)-aspidospermine (137). The stereochemistry of the 19-hydroxy group was determined by examining the rotatory dispersion of the methylxanthate derivative of 135, which indicated the -stereochemistry for this center, as shown (62). 

The episulfides (82), (83), (84), and (87) react readily with potassium methylxanthate in methanol at room temperature to give crystalline tri-thiocarbonate derivatives. Assuming attack at the primary position,the reaction from (84) may be formulated as follows. 

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