Aminals generation, boron trifluoride etherate

Methoxycarbonylformonitrile oxide is smoothly generated by 3 -elimination of methanol from E -N-methoxy-N-(methoxycarbonylmethylene)amine N-oxide, MeC>2CCH=N(0Me)0, in the presence of a catalytic amount of boron trifluoride etherate (96). 

Barton oxidation was the key to form the 1,2-diketone 341 in surprisingly high yield, in order to close the five-membered ring (Scheme 38). The conditions chosen for the deprotection of the aldehyde, mercuric oxide and boron trifluoride etherate, at room temperature, immediately led to aldol 342. After protection of the newly formed secondary alcohol as a benzoate, the diketone was fragmented quantitatively with excess sodium hypochlorite. Cyclization of the generated diacid 343 to the desired dilactone 344 proved very difficult. After a variety of methods failed, the use of lead tetraacetate (203), precedented by work performed within the stmcmre determination of picrotoxinin (1), was spectacularly successful (204). In 99% yield, the simultaneous formation of both lactones was achieved. EIcb reaction with an excess of tertiary amine removed the benzoate of 344 and the double bond formed was epoxidized with peracid affording p-oxirane 104 stereoselectively. Treatment of... 

Diazotization in organic solvents allows solid diazonium salts to be isolated. Diazotization can be carried out using an ester of nitrous acid, such as pentyl nitrite, in a solvent such as acetic acid or methanol. A procedure has also been described for isolating diazonium tetrafluoroborates, in excellent yield, by carrying out the diazotization with boron trifluoride etherate and f-butyl nitrite in ether or dichlorometh-ane at low temperature. Another method for the preparation of a variety of diazonium salts in a nonaqueous medium makes use of the chemistry of bis(trimethylsilyl)amines (8). These compounds react in dichloromethane with nitrosyl chloride and other nitrosating agents which are generated in situ. Thus, benzenediazonium chloride was isolated (96%) from bis(trimethylsilyl)aniline. 

The reduction can also be achieved by utilizing in situ generated BHj THF (from sodium borohydride and boron trifluoride etherate). The scope of this reaction includes the synthesis of novel 3-chroman-amine derivatives (60 equation 33). This stereoselective reaction proceeds via the hydroxylamine intermediate only c/s-2-aryl-3-amino derivatives are obtained. 

A new synthetically useful procedure for the preparation of arenediazonium tetrafluoroborates from the corresponding amines involves the in situ generation of nitrosyl fluoride (Scheme 53). The diazotization reactions are performed under mild conditions in an anhydrous solvent, preferably methylene chloride. Excess boron trifluoride etherate traps the alcohol and water produced, and the diazonium salts are precipitated from the reaction medium and isolated by simple filtration. 

Alkyl nitrites can also be used in anhydrous media and, for example, 4-hydroxyben-zenediazonium tetrafluoroborate is isolated in 89% yield after diazotization with isopcntyl nitrite, hydrogen fluoride and boron trifluoride in ethanol/diethyl ether.96 Diazotization can also be performed in dichloromethane or ethers (diethyl ether, tetrahydrofuran, 1,2-dimethoxyethane) with terl-butyl nitrite and boron trifluoride-diethyl ether complex which generates nitrosyl fluoride in situ.229 Excess boron trifluoride is used to trap water and tert-butyl alcohol, so that the reaction can be considered as being performed under complete anhydrous conditions. Yields are higher in dichloromethane, but 1,2-dimethoxyethane is preferred for less soluble amines. This procedure has been successfully applied to the synthesis of mono-and difluorobenzo[c]phcnanthrenes.230... 

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