Boron trifluoride construction materials

Enantiospecific syntheses of amino derivatives of benzo[ ]quinolizidine and indolo[2,3- ]quinolizidine compounds have also been achieved via A-acyliminium ion cyclization reactions, as an alternative to the more traditional Bischler-Napieralski chemistry (see Section 12.01.9.2.2). One interesting example involves the use of L-pyroglutamic acid as a chiral starting material to construct intermediates 240 via reaction with arylethylamine derivatives. Diisobutylaluminium hydride (DIBAL-H) reduction of the amide function in 240 and subsequent cyclization and further reduction afforded piperidine derivatives 241, which stereoselectively cyclized to benzo[ ]quinolizidine 242 upon treatment with boron trifluoride (Scheme 47) <1999JOC9729>. 

The synthesis [25, 26] began with 7-bromo-l-tetralone (7), available in multigram quantity in three steps from succinic anhydride and bromobenzene [27, 28] (Scheme 1). Treatment of 7 with an aromatic aldehyde and base produces the corresponding benzylidene (e.g., 8) which couples with 7 in boron trifluoride etherate to form a pyrylium salt. This pyrylium salt is not isolated but treated with ammonia to afford functionalized spacer 9. As seen in Scheme 1, the attachment of anthracene [29] or acridine chromophores occurs in a single step. Alternatively, stepwise attachment of the aromatic chromophores allows construction of molecular tweezers 12 and 13 carrying different chromophores. With respect to efficiency, tweezers 10 and 11 are synthesized in six steps from inexpensive starting materials with overall yields of 11% and 14%, respectively. 

In consideration of conceivable strategies for the more direct construction of these derivatives, nitriles can be regarded as simple starting materials with which the 3+2 cycloaddition of acylcarbenes would, in a formal sense, provide the desired oxazoles. Oxazoles, in fact, have previously been obtained by the reaction of diazocarbonyl compounds with nitriles through the use of boron trifluoride etherate as a Lewis acid promoter. Other methods for attaining oxazoles involve thermal, photochemical, or metal-catalyzed conditions.12 Several recent studies have indicated that many types of rhodium-catalyzed reactions of diazocarbonyl compounds proceed via formation of electrophilic rhodium carbene complexes as key intermediates rather than free carbenes or other types of reactive intermediates.13 If this postulate holds for the reactions described here, then the mechanism outlined in Scheme 2 may be proposed, in which the carbene complex 3 and the adduct 4 are formed as intermediates.14... 

Dry boron trifluoride does not react with the common metals of construction, but if moisture is present the acidic hydrates formed (BF3 H2O and BF3 2H2O) can corrode many common metals rapidly. Consequently, lines, pressure regulators, and valves in boron trifluoride service must be well protected from the entrance of moist air between periods of use. Cast iron must not be used because active fluorides attack its structure. If steel piping is used for boron trifluoride, forged-steel fittings must be used instead of cast-iron fittings. Stainless steel. Monel, nickel, and Hastelloy C are good materials of construction. 

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