7?-TsOH

Scheme 71. Reaction conditions i, Zn(BH4>2 ii, DHP-p-TsOH DIBAH-PhMe LDA-f-BuOAc iii, 7-TsOH-MeOH, p-TsOH-PhH iv, MeNOj-Triton B v, TiCIa-EtsN vi, cone HCl-CH2CI2, vii, HSCH2CH2SH-BF3-Et20 viii, LDA-MeOC(Me2)OCH2COOMe, CSA-CH(OMe)3, DMS0-DCC-Py-CF3C00H-Et20 ix, Zn(BH4)2, PhCOCl-DMAP-Py x, HgO-HgClj-aq MeCN Zn(BH4)2 xi, PhSeCN-Bu3P xii. PrSLi-HMPA.

Etherification. When cholesterol (1) is heated for several hours in dimethyl phosphite, cholesteryl methyl phosphite (2) is obtained as the main product. However, if /7-TsOH is present, 3/f-methoxy-A5-cholestene (3) is obtained as the main product in about 60 % yield. Cholestanol is converted by this method into 3/ -methoxycholestane in 60-70% yield. 

TsOH, benzene, reflux, 30 min, 76% yield. A f-butyl ester is stable to the conditions needed to convert an cx,(3-unsaturated ketone to a dioxolane (HOCH2CH2OH, TsOH, benzene, reflux). TsOH with microwave heating has also been used on a few trivial esters. ... 

Hydroxyindazoles. A soln. of 2-(4-methoxyphenyl)-4-phenyl-4-(2,4,6-trinitro-phenyl)-5(4//)-oxazolone in methanol refluxed for 3 h in the presence of a little /7-TsOH - 4,6-dinitro-3-phenyl-l//-indazol-l-ol. Y 78%. F.e. and 6-nitro-lff-pyrazolo 4,3- pyridin-l-ols from the corresponding dinitropyridyl analogs s. M. D Anello et al., Chem. Ber. 121, 67-73 (1988). 

A little /7-TsOH added in one portion at room temp, to a mixture of tetramethyl orthocarbonate and rra 5 -2-mercaptocyclohexanol in methylene chloride, a few drops of NEt3 added after a few min, and the mixture evaporated - cyclohexene sulfide. Y 78%. The method is simple, short, and generally applicable. F.e. and catalysts s. T. Takata, T. Endo, Bull. Chem. Soc. Japan 61, 1818-20 (1988). 

Pinguisenol (462), an important precursor of the co-metabolites (461, 466,469,477,478) was isolated from Porella vernicosa complex (50). Facile dehydration of (462) with/7-TsOH gave two conjugated hydrocarbons (461, 490), whose ozonolysis without separation followed by methylation afforded the dimethyl ester (491), which was also produced by similar treatment of (466) (Scheme 59) (50). The absolute configuration of the hydroxyl group was based on the paramagnetic shift of H-15 in the H-NMR spectrum and facile dehydration of (462). 

The most frequently employed acid catalyst is TfOH Trifluo-romethanesulfonic Acid, or triflic acid), but others that have been used include BF3 0Et2, TMSOTf, /7-TsOH, and TFA. Camphorsulfonic acid, pyridinium p-toluenesulfonate, and trityl perchlorate have also been investigated in conjunction with the more reactive 4-methoxybenzyl trichloroacetimidate. ... 

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