Desulfurization thioketals

The enone (137) was allowed to react with MeMgBr—Cu2I2 complex at 0 °C to furnish the keto-ester (138), which was then reduced by NaBH4 to afford the lactone (139). On the other hand, consecutive thioketalization, desulfurization and reduction of the ester (138) gave the alcohol (140), which was allowed to react with pTsCl in pyridine to afford (141) after smooth and concomitant fragmentation. The compound (141) was converted to 5-epi-kessane (142) and dehydrokessane (143) 47). 

WOLFF-KISHNER, THIOKETAL DESULFURIZATION, OR CLEMMENSON REDUCTION OF A KETONE KETHYDROGENOLYSIS KETONE PATH 1 PRIORITY 50 CHARACTER INTRODUCES GROUP... 

Thioketal desulfurization Additive reduction with elimination... 

Thioketal desulfurization with elimination Additive reduction 0.043 22.113 0.319 -2 1 Hyperhypsic... 

A variety of different routes to cyclobutane systems involving non-photochemical means has also been devised. Acid catalyzed rearrangement of fused cyclopropyl ether (61) by Wenkert et al. (97, 98) afforded the bicyclic dione (62), which through a thioketal-desulfurization process and treatment with hydroxylamine yielded oxime (58) (Scheme 12). The shortest synthesis of grandisol is that reported by Billups et al. (Scheme 13) (99). Dimerization of isoprene in the presence of a zero-valent bis-cyclooctadienyl-nickel-phosphite complex gave the cis-cyclobutane diolefin (65), which could be separated in 12—15% yield from the complex product mixture by low-temperature distillation. Selective hydroboration and oxidation afforded grandisol. 

An older method,which also retains the double bond in its original location, utilizes the fact that thioketal formation from A" -3-ketones does not induce bond migration. Subsequent desulfurization with Raney nickel gives the A" -olefin (see section XI-D). 

The thioketal (155) is formed from 3jS-hydroxypregn-5-en-20-one acetate (154) and is desulfurized by treatment with Raney nickel. ... 

Dimethyl ketals and enol ethers are stable to the conditions of oxime formation (hydroxylamine acetate or hydroxylamine hydrochloride-pyridine). Thioketals and hemithioketals are cleaved to the parent ketones by cadmium carbonate and mercuric chloride. Desulfurization of thioketals with Raney nickel leads to the corresponding methylene compounds, while thioenol ethers give the corresponding olefin. In contrast, desulfurization of hemithioketals regenerates the parent ketone. ... 

Deuteration at C-12 by desulfurization of 12-thioketals with Raney Nickel preparation of the ethylene thioketal, 172 desulfurization of the mercaptal, 173... 

Carbonyl groups can be converted to methylene groups by desulfurization of thioketals. The cyclic thioketal from ethanedithiol is commonly used. Reaction with excess Raney nickel causes hydrogenolysis of both C—S bonds. 

The keto function is frequently deoxygenated via Ra-Ni-mediated desulfurization of thioacetals 412 14 That is, the ketone group can be removed from compound (36) by thioketalization followed by desulfurization with Ra-Ni (81 mmol Ni/mmol) in MeOH for 20 minutes (Scheme 4.118).415... 

Reductions of keto esters to esters are not very frequent. Both Clemmensen and Wolff-Kizhner reductions can hardly be used. The best way is desulfurization of thioketals with Raney nickel (p. 130). Thus ethyl acetoacetate was reduced to ethyl butyrate in 70% yield, methyl benzoylformate (phenylglyoxy-late) to methyl phenylacetate in 79% yield, and other keto esters gave equally high yields (74-77%) [82J]. 

Deuteration at C-12 by Desulfurization of a 12-Thioketal with Raney Nickel... 

Thioketals, unlike ordinary ketals, are formed readily from ketones and thiols (RSH) in the presence of acid catalysts. The desulfurization procedure usually goes well, but the product is rather difficult to separate by extraction from the large excess of Raney nickel required for optimum yields. 

The total synthesis of ( + )-Erysotramidine (2) has been described by Ito et al. (137) starting from the amide (174) (Scheme 39). After O-mesylation to 177, base-catalyzed reaction gave the cyclopropane derivative (178) which with zinc in acetic acid was reduced to 179, which was identical to the product (135) of O-methylation of 172. Conversion of 178 to the thioketal (180) was followed by reaction with phenylselenyl chloride. A mixture of two compounds, 181 and 182, was produced the former could be transformed quantitatively to the latter. Finally, treatment of 182 with silver nitrate in methanol gave 183, which was then desulfurized to yield erysotramidine (2). 

For C-extension in the (5/ )-hydroxyhexanal case it was found (58), that the pyranoid lactol form 41 is so stable that reaction with Wittig ylides cannot be effected under standard conditions. Thus, the acylic form had to be elaborated, which was effected by thioketalization, acetylation (41 - 43) and desulfurization, the resulting 5-O-blocked hexanal 44 then smoothly affording the olefin 45. Liberation of the hydroxyl function and ensuing lactonization... 

The thioketal-containing hydroxyl hnker 1.31 (88) was prepared in three steps from Merrifield resin and used to support carboxyhc acids. The stable resin-bound intermediate is activated via desulfurization, with either Hg(C104)2 or HIO4, and the resulting hnker is photolyzed in standard conditions to give the pure, released acid. 

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