Boron Trifluoride Etherate rearrangements

Geometrically defined a/ -epoxysilanes have been shown (6) to undergo a highly stereoselective rearrangement to silyl enol ethers (see also Chapter 15). This rearrangement is catalysed by boron trifluoride etherate, and seems to involved-opening of the epoxysilane, as shown ... 

Very interesting rearrangements of fused bicyclic acetals (243), which occur by the action of boron trifluoride etherate, were found and studied in detail by Chlenov et al. (401) (Scheme 3.161). 

Another interesting observation in this study is the boron trifluoride etherate-catalyzed rearrangement of tri-ir-methane systems 141 that afford the corresponding cyclopentenes 142. These reactions can be considered as the first examples of tri-ir-methane rearrangements in the ground state. Interestingly, compounds 141 only undergo conventional di-TT-methane reactions on irradiation. The mechanism shown in Scheme 25 is proposed to account for this novel reaction [79]. 

A variety of ketoxime ethyl carbonates undergo the Beckmann rearrangement in excellent yields (74-99%), upon treatment with 1 equivalent of boron trifluoride etherate at room temperature (equation 91). 

Boron trifluoride etherate was used in conjunction with the reducing agent borane to rearrange aromatic O-triisopropylsilyl ketoximes to cyclic and acyclic aniline derivatives. The steric hindrance of the substituents on the silicon atom, the size of the aliphatic ring and the presence of alkoxy substituents on the aryl group played important roles in the aniline formation. 

Similarly, boron trifluoride etherate in hot acetonitrile acts on enone 416 to promote its rearrangement to 417... 

Methyl trisnorshionanoate (126) has been synthesized from friedelan-19a-ol (127) via the ring-contracted intermediate (128) and the trisnor-ketone (129). Irradiation of (129) in methanol afforded the desired ester (126). An investigation of the effect of solvent on the backbone rearrangement of 3/8,4/3-epoxyshionane (130) by boron trifluoride etherate has shown that nucleophilic solvents tend to interrupt the rearrangement at an early stage (see Vol. 6, p. 130). 

The orf/to-formylation of 2-aminopyridines can be effected via the rearrangement of the azasulfonium salt prepared from a 2-aminopyridine, 1,3-dithiane, f-butyl hypochlorite and sodium methoxide (74CC685). The crude sulfilimine (815) was refluxed in f-butanol containing potassium f-butoxide to yield the dithioacetal (816). Hydrolysis of (816) with mercury(II) oxide/boron trifluoride etherate gave the aldehyde (817 Scheme 191). This method should be applicable to the formylation of other heterocyclic amines. 

The oxidative rearrangement of chalcones is a valuable route to isoflavones which has been thoroughly investigated. Initially, the conversion was achieved in two distinct steps. Epoxidation of a 2 -benzyloxychalcone, carried out by conventional techniques, is followed by treatment with a Lewis acid, such as boron trifluoride etherate, which brings about the rearrangement. 

Polyprenylquinones (cf 9, 499-500).5 Coenzyme Qn (3) can be obtained by rearrangement of the polyprenyl aryl ethers 1 with boron trifluoride etherate to the hydroqtiinones (2) followed by oxidation. 

Diazo ketone cyclizathn. Some years ago Mander and his group1 demonstrated that the protonated diazomethylcarbonyl group can initiate cyclizations in unsaturatcd systems. In the case of phenolic diazo ketones, formation of spirodienones can predominate over competing side reactions (dienone-phenol rearrangement). Tetra-fluoroboric acid or boron trifluoride etherate can be used, but trifluoroacetic acid is usually the acid of choice. 

Oxetanes having ether groups in a side chain (21) undergo rearrangement in the presence of boron trifluoride etherate to give... 

Allyloxy-l,2,3,6-tetrahydropyridine 241 undergoes Claisen rearrangement in cymene to give 4-allyl-3-oxo-piper-idine 242. The boron trifluoride etherate-mediated Claisen rearrangement gives only the 2-allyl isomer 244 due to the initial isomerization of tetrahydropyridine 241 into 1,2,3,4-tetrahydropyridine 243 by the Lewis acid (Scheme 61) <1999S1814>. 

In a similar way, O-methylflavinantinol (59) rearranges under the action of boron trifluoride etherate to give, via the neospirinedienone 60, the iminium salt 61, whose catalytic reduction produces laurifonine (4) in excellent overall yield (55) (Scheme 15). Laurifonine (4) has also been obtained by TFA-promoted dienol-benzene rearrangement of 59 followed by reduction with sodium borohydride 24). 

The reaction of linalool with boron trifluoride etherate has been re-examined no pinenes or camphene were obtained.146 Dehydrolinalool reacts with methyl iso-propenyl ether under acidic conditions by Claisen rearrangement to give the allene (58).147 Further papers in this section include reaction of monoterpenoid alcohols with paraformaldehyde-acetic anhydride-sodium acetate,148 rearrangement of the alcohol (47 X = OH) to the oxabicycloheptane (59) and the ketone (60),149 and the rearrangement of a typical monoterpenoid vicinal hydroxy-ester to an epoxide.150... 

A triterpenoid oxide, dendropanoxide (138), has been synthesized from friedelin via the 3(3,4(3-epoxide (139) of friedel-3-ene.109 On treatment with boron trifluoride etherate the epoxide (139) yielded dendropanoxide (138) together with the rearranged products (140) and (141). 

The acid-catalysed decomposition of the adducts (291) and (292 R = 0) derived from the parent steroidal 5,7-dienes furnishes a novel route to 4,4-dimethylcholesta-5,7,14(15)-trien-3-one (90%) and cholesta-4,6,8( 14)-trien-3-one respectively.165 It has also been noted that reaction of the adduct (292 R = /3-OAc,H) with boron trifluoride etherate gives an oxidative rearrangement to the anthrasteroid (293)... 

A version of this type of oxidation, which used a combination of lead tetraacetate and boron trifluoride etherate, allowed the transformation of enamines of cyclic ketones into esters of the corresponding contracted ring, as in the classical Favorskii rearrangement of a-haloketones under basic conditions72 (Scheme 51). 

The selenosulfonates (26) comprise another class of selenenyl pseudohalides. They are stable, crystalline compounds available from the reaction of selenenyl halides with sulftnate salts (Scheme 10) or more conveniently from the oxidation of either sulfonohydrazides (ArS02NHNH2) or sulftnic acids (ArS02H) with benzeneseleninic acid (27) (equations 21 and 22). Selenosulfonates add to alkenes via an electrophilic mechanism catalyzed by boron trifluoride etherate, or via a radical mechanism initiated thermally or photolytically. The two reaction modes produce complementary regioselectivity, but only the electrophilic processes are stereospecific (anti). Similar radical additions to acetylenes and allenes have been reported, with the regio- and stereochemistry as shown in Scheme 11. When these selenosulfonation reactions are used in conjunction with subsequent selenoxide eliminations or [2,3] sigmatropic rearrangements, they provide access to a variety of unsaturated sulfone products. 

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