Methanesulfonic acid 0-silylations with

An hDA reaction of the thermally generated (trialkylsilyl)vinylketene 888 with diethyl ketomalonate furnishes the 5,6-dihydropyran-2-one 889 in excellent yield. Protodesilylation of the cycloadduct 889 is achieved in quantitative yield upon its exposure to methanesulfonic acid (Scheme 244). A photochemical Wolff rearrangement of the silyl diazo compound 890 can also be used to generate an intermediate diene for reaction with diethyl ketomalonate to afford the 5,6-dihydropyran-2-ones 891 (Equation 358) <19990L641>. 

Besides the ene oxide and the silyl enol ether discussed above the enamine derivatives of cyclohexanone can be used as a cyclohexanone equivalent as well in these novel annelation procedures [151]. Alkylation of the enamine 108 with l,5-bis(2 -bromoethyl)naphthalene (109) afforded the expected diketone 110 in 80% yield (see Scheme 51). Cyclodehydration of 110 with methanesulfonic acid... 

TfOH protonates naphthalene at room temperature to give a complex mixture of products. TfOH promotes aldol reaction of silyl enol ethers with aldehydes and acetals, leading to new C-C bond formation (eq 15). TfOH con ietes well with other reagents employed for the aldol reaction, while Methanesulfonic Acid does not afford any product. 

Trimethylsilyldiazomethane smoothly reacts with (trialkyl-silyl) vinylketenes, ultimately prepared from diazoketones, in a net [4+1] annulation process to afford 2-trialkylsilylcyclopentenones in good to excellent yields (eq 68). The products are readily desi-lylated by treatment with methanesulfonic acid in methanol. An analogous reaction with (trialkylsilyl)arylketenes gives trialkyl-silyl-2-indanone derivatives (eq 69). ... 

In Zhu s approach, the advanced intermediate 34 was subjected to Swern oxidation and tetra- -butyIammonium fluoride (TBAF)-mediated deprotection of the silyl ether, yielding a mixture of aldehyde 35 and hemiaminal 36. Treatment of this mixture with 0.01% v/v methanesulfonic acid in dichloromethane initiated a process consisting of acyhminium formation, deprotonation, and domino P-elimination/cydization. Thus, dehydration of hemiaminal 36 initially led to iminium ion 37, which formed enamide 38 under loss of a proton. Elimination of the sulfur side chain then resulted in conjugated iminium ion 39, which was set up to undergo a phenolic Mannich cych2ation. 

Other useful dehydrating agents are dimethylaminosulfur trifluoride (DAST), methyl A-(triethylammoniosulfonyl)carbamate (Burgess salt), acetic anhydride, oxalyl chloride, and phosphorous oxychloride, each one in combination with triethylamine (89). Dehydration of O-silylated hydroxamic acids using trifluoro-methanesulfonic anhydride and triethylamine under mild conditions also gave nitrile oxides, which in the presence of olefins led to the formation of 2-isoxazolines in moderate to good yields (90). In view of the less readily available starting materials, this method probably will be of limited use. 

The reaction with acetals does not always need the acetal itself to be synthesized it can be made in the same flask by mixing the allylsilane, the aldehyde, the silyl ether of the alcohol, and a catalytic amount of an acid such as diphenylboryl trifluoro-methanesulfonate, trimethylsilyl trifluoromethanesulfonate, or fluorosulfuric acid. ... 

---