Ring expansion oxidative

Oxidative ring expansion occurs when the hydrazine (175) is treated with manganese dioxide (73TL4091). Diphenylcyclopropenone (176) reacts with ammonia and methylamine to give /3-lactams (69BCJ1777). Initial attack could occur at either C-1 or C-2 of the cyclopropenone. 

Conversion of furfuryl alcohol derivatives 48 to pyranones 49 (Achmatowicz oxidative ring expansion) is employed in the synthesis of spiroketal moiety of a natural product and cyclopentenones <00TL6879>. 

Silyloxypropanols (314) derived from lactones undergo oxidative ring expansion to afford higher homologous o ,/3-unsaturated lactones (316) (90TL197). The reaction proceeds through the fluoride ion assisted formation of intermediate 315, which on reductive elimination of F " species yields the product (Scheme 80). 

Racemic argemonine (5) has been synthesized from the readily available tetrahydro-6,12-methanodibenz[c,/Iazocine (74) (120-122) through a sequence involving a Stevens rearrangement and in an overall yield of 53% from 74 (Scheme 11) (123). Hofmann degradation of 74 furnished the cxo-methylene compound 75 (120,122). An oxidative ring expansion of 75 afforded ketone 76, which was then reduced to secondary alcohol 77. A transannular reaction, effected by acetic acid-acetic anhydride, resulted in the formation of the tetra-... 

Lead tetraacetate oxidation of 3-substituted 1- and 2-aminoindazoles 6 and 7, has been shown by Rees to result in rapid and almost quantitative conversion into 4-substituted 1,2,3-benzotriazines (8, R = Me, OMe, Ph). 1,2,3-Benzotriazine (8, R = H) was prepared similarly, and for the first time, by carefully controlled oxidation of either 1- or 2-aminoindazole (6, 7, R = H). These remarkable ring expansion reactions, which apparently proceed via the formation of intermediate nitrenes, appear to be a direct and logical extension of original work by Bamberger and Goldberger in 1898 on the oxidative ring expansion of... 

Oxathiane 2-oxides aie fonned by the oxidative ring expansion of 2-alkylthio-2-benzylthiolane 1-oxides brought about by [bis(trifluoroacetoxy)iodo]benzene. That the reaction is only successful with the (lR )-diastereoisomeis is attributed to chelation between the nucleophilic S and O atoms and the hypervalent iodine <99EJ0943>. A diazo-mediated thiolane ring expansion is the key step in a synthesis of the acenaphtho-[U-b][l,4]oxathiine system <99JCS(P2)755>. 

Some of the ring expansion reactions discussed in Section 2.03.3.3.1 can be extended to five-membered heterocycles containing two or more heteroatoms. Reaction of imidazoles and pyrazoles with dichlorocarbene, for example, gives chloropyrimidines together with small amounts of chloro-pyrazines or -pyridazines, and oxidative ring expansion of 1-aminopyrazole with nickel peroxide gives 1,2,3-triazine (this, in fact, constitutes the only known synthesis of the unsubstituted triazine). There are, however, a number of interesting and useful transformations which are unique to five-membered polyheteroatom systems. 

The JV-amino derivatives 269 underwent oxidative ring expansion to tri-azine 270. Analogous pyrazolo[3,4-c]pyridines, however, gave only intractable products. The latter system lacked the peri interaction necessary for repulsion of nucleophilic attack at the 4-position of the triazine ring.157... 

This homologous Baeyer-Villiger type oxidative ring expansion represents a conceptually new protocol illustrating the substantial value of one-pot, four-component annulations as a flexible and simple new synthetic method. 

The Baeyer-Villiger enzyme, cyclohexanone monooxygenase (CHMO), has been applied to the oxidative ring expansion of m-2,6-dialkylperhydropyrans to afford 28 with very high yields and ee s, when R = methyl or ethyl (Scheme 11) <2001JM0349, 2003SL1973>. 

Similarly, exposure of stannyl lactol to PhI(OAc)2 4 led to oxidative ring expansion yielding the trans unsaturated lactone stereoselectively, presumably via 1,4-fragmentation induced by reductive elimination of iodobenzene [Eq. (46)] [83]. 

While this approach to ring expansion yielded the desired product 252 in an overall yield of 47%, photochemical (room lights) and thermal sensitivity (room temperature) of the dibromomethyl intermediates inspired the development of an alternate approach. As shown in Scheme 21, decalenone 249 could be smoothly methylenated with Tebbe reagent to provide diene 255 in excellent yield. Clean and reproducible oxidative ring expansion could then be effected with thallium trinitrate to furnish enone 252 in 62% overall yield (unoptimized). 

The above results [63] demonstrated the capacity of resting cells entrapped in polyethyleneimine-barium alginate to perform repeated oxidative ring expansion of penicillin G to DAOG. Product formation rate was higher with free than... 

Immobilized cells of S. clavuligerus NP1, entrapped on a polymeric matrix, were able to perform oxidative ring expansion of penicillin G into DAOG. Cells entrapped in polyethyleneimine barium alginate (1.5%) were able to sustain activity for at least four 2-hr cycles, whereas free resting cells were inactive after the second cycle. 

Oxidative ring expansion of enamides.2 A general route to tetrahydro-3-benz-izepine-2-ones involves oxidative ring expansion of enamides of dihydroisoquino-Imes, which are easily formed by reaction with anhydrides or acid chlorides. The... 

The biosynthesis of IPN (Schemes 48 and 49) is common to the production of penicillins and cephalosporins. In some organisms, exchange of the L-a-aminoadipoyl side chain of IPN results in the formation of the hydrophobic penicillins (mainly G and V) this step is catalyzed by a penicillin acyltransferase. In others, epimerization of the IPN L-configured side chain to the D-configured side chain of penicillin N <2000T7601>, catalyzed by IPN epimerase, followed by oxidative ring expansion leads to the cephalosporin family . 

In order to realize this basic idea, we started with the ( -protected 3(2-furyl)-2-propenol13 23 obtained from 11. Thus, dihydroxylation of ( )-23 gave the diol ( )-24 which was transformed into 6-alkoxymethyl isolevoglucosenone ( )-26 via 25 by sequential oxidative ring expansion and cyclization. ( )-26 was then transformed into ( )-27 and ( )-28 for lipase-mediated resolution (Scheme 7). 

Oxidative ring expansion of tetrahydrofurans can be promoted by silver(i) salts to afford tetrahydropyrans in moderate yield (Equations 161 and 162) <1996J(P1)413, 1996TL213>. Zinc salts can also effect this transformation <1999TL2145>. 

Fig. 32 Vanadyl acetylacetonate-catalyzed oxidative ring expansion of bicyclic cyclopropanols...

The preparation of 3,1-benzoxazepines by photochemical isomerization of quinoline A-oxides constitutes a rather general entry into this class of seven-membered heterocycles. Since the structure of the photoisomer of 2-phenylquinoline A-oxide was first recognized as 2-pheny 1-3,1-benzoxazepine by Buchardt et al.,3 the scope of this method for oxidative ring expansion of six-membered heterocyclic A-oxides to 1,3-oxazepines has been extensively explored.4 For example, irradiation of 2-cyano-, 2-phenyl-, and 2-methoxyquinoline A-oxides affords the corresponding 2-substituted 3,1-benzoxazepines in 70-90% yield.5 However, isolation of the moisture-sensitive parent compound was only recently accomplished in the submitters laboratories.6... 

Explain the regioselectivity obtained in steps j and s. Rationalise the stereoselectivity of these oxidative ring expansion steps. 

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