Rearrangements boron trifluoride catalyzed

The addition of HN3 to alkenes in the presence of Lewis acids also involves carbocationic intermediates. This is nicely illustrated by the boron trifluoride-catalyzed addition of HN3 to a- or (J-pinene in which carbocation rearrangements are observed (equation 182).263... 

The stereoselective formation of cw-fused dihydropyrans and the steric requirements for the reaction of the norbornenyl examples support a concerted mechanism. However, boron trifluoride catalyzed rearrangements of mixtures of endo- and exo-compounds to bicyclic pyrans, i.e., stepwise reactions through cationic intermediates, are also possible38. 

Boron trifluoride catalyzes the condensation of phenol and propylene to isopropyl phenyl ether and the subsequent rearrangement of this compound to o-isopropyl phenol. This rearrangement of an aryl alkyl ether is similar to the Fries reaction of phenolic esters (method 209). 

Kometani, T, Kondo, H, Fujimori, Y, Boron trifluoride-catalyzed rearrangement of 2-aryloxyte-trahydropyrans a new entry to C-arylglycosidation, Synthesis, 1005-1007, 1988. 

To complete the synthesis, the required 13a,17a-epoxide 187 was best obtained via the chlorohydrin 186 (and/or the isomeric 13a-hydroxy-17P-chloro structure) rather than by direct epoxidation, which gave mainly the -epoxide. Boron trifluoride catalyzed rearrangement of 187 then gave ( )-estrone 26 in 22% overall yield from 182 (R = H). 

Sterols and Related Products - The Westphalen rearrangement of some JP-substituted-bp-acetoxy-5Q -hydroxycholestanes has been studied and the products and kinetics shown to be dependent on the nature of the 5-substituent. The conformations of the products were studied and in the case of the Westphalen-Lettre derivatives it was shown that ring B is in the boat conformation. The boron trifluoride catalyzed cleavage of 30 -acetoxy-4a,5-epoxy-5Ci -cholestane afforded the expected backbone rearrangement products containing a 15(17) doable bond." A similar experiment with 5Q -acetoxy-5,ba-epoxy-5o -cholestane yielded some of the rearranged derivative XW after a reaction time of 25 seconds, along with the e q)ected isomer " ... 

Berti, G., a. Marsili, J. Morelli, and A. Mandelbaum Boron Trifluoride-catalyzed Rearrangements of Some Tetrasubstituted Neotriterpene Epoxides - II. Hopene-II Oxide and Its Analogue in the A B-Neoallobetulin Series. Tetrahedron 27, 2217(1971). 

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]. 

This procedure for the acetylation of methyl alkyl ketones to form /3-diketones is a modification5 of an earlier procedure, which used boron trifluoride gas as the catalyst.6 3-n-Butyl-2,4-pentanedione has also been prepared by the acetylation of 2-heptanone catalyzed with boron trifluoride gas,7 by the thermal rearrangement of the enol acetate of 2-heptanone,7 and by the alkylation of the potassium enolate of 2,4-pentanedione with n-butyl bromide.8... 

Cycloadduct 212 is converted into the dihydrobenzofuran 213 when treated with boron trifluoride diethyl etherate in good yield <2000JA8155>. The formation of the dihydrobenzofuran proceeds by an initial ring opening followed by a subsequent dehydration and acid-catalyzed cyclopropyl ketone rearrangement (Equation 142). 

Pure isomerizations have been observed in Lewis acid catalyzed thermal1,2 7 and photochemical3 4 rearrangements, but incorporation of the electrophile occurs with acid fluoride-boron trifluoride.5 6 Examples are collected in Table 2. 

The ring enlargement of 2,3-divinylcyclobutanones has been catalyzed by boron trifluoride or trifluoroacetic acid.110 This acid catalysis, however, usually gives lower yields than the thermal reaction and favors rearrangement to five- or six-membered rings. 

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. 

Diethyl phosphorocyanidate adds to a,/J-unsaturated aldehydes or ketones in the presence of lithium cyanide in a 1,2-fashion28. Boron trifluoride-diethyl ether complex catalyzed rearrangement of these allylic phosphates shows high E selectivity (>85 15) for the adducts derived from aldehydes and Z selectivity (>90 10) for ketone adducts. The selectivity of the rearrangement can be explained by assuming a chairlike transition state, in which the sterically more demanding x-substituent occupies the quasi-equatorial position. The steric requirement decreases in the order of R1 > CN > H. Thus, the cyano substituent occupies the quasi-equatorial position in the aldehyde-derived adduct (R1 = H), but the quasi-axial position in the ketone-derived adduct (R1 = CH3, C6H5). 

When 2-crotyloxypyridine (63) was heated under similar conditions, however, the results were catalyst dependent (Scheme 5). With boron trifluoride-diethyl ether as catalyst, an 82 18 mixture of the pyridones 64 and 65 was obtained, whereas the H2PtCl6-catalyzed reaction gave a [3,3]-rearrangement product, the pyridone 66 (68JOC4560). Interestingly, when Pt(PPhj)4 was used as catalyst, 63 and 2-(l-methylallyloxy)pyridine (67) both gave the same 14 86 mixture of the pyridones 64 and 66, suggest-... 

Medium and large rings can be made via cationic intermediates which are usually generated by treatment of suitable precursors with acid. Reactions of this class are perchloric acid catalyzed rearrangements of bicyclo[n.l.0]alkan-2-ols (n > 5), solvolysis of the corresponding esters, boron trifluoride-diethyl ether complex catalyzed cleavage of epoxides, and tri-fluoroacetic acid catalyzed reactions of 7-(methylsulfanyl)bicyclo[n.l.0]alkanes. 

The most versatile method involves rearrangement of an acylated hydroxymethyloxetane catalyzed by boron trifluoride etherate (13. 17). The triols are readily converted to the required hydroxymethyl oxetanes via pyrolysis of the carbonate esters (18) and then to the acylated hydroxymethyloxetanes with acid chlorides. 

In some cases, the stcric constraints of the zeolite pores can enable or impede secondary reactions. A good example is found in the rearrangement of benzyl allyl ethers over 11-Beta zeolites. In the normal course of this reaction, catalyzed by H-Bcta or boron trifluoride etherate, the benzyl allyl ether is transformed to a 4- arylbutanal, probably via a five- membered ring intermediate e.g. methallyl 2,5-dimethoxybenzyl ether ... 

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