Photochemical ring-contraction

Generation of some small-ring cyclic acetylenes has been achieved either by matrix isolation studies or by trapping and oligomerization experiments157-160. However, no example of the isolation of cyclohexyne has been reported. While the six-membered cyclic acetylene 156a was prepared for the first time by the ring-contracted photochemical... 

Photo-Wolff rearrangement in cyclic o-diazo ketones is accompanied by ring contraction. Photochemically induced conversion of the a-dlazocyclopentanone (65) into the cyclobutane... 

Aziridinimine derivatives are also accessible by the photochemically induced extrusion of nitrogen and ring contraction of simple alkylidene tetrazolines (424) <75AG(E)428). Thus, Pyrex-filtered photolysis of (424) at -60 °C quantitatively afforded the aziridinimines (425). 

Several ring contraction routes to /3-lactams have been developed. One of the most important is the photochemical Wolff rearrangement of 3-diazopyrrolidine-2,4-diones (178), which provides a general route to 3-carboxyazetidin-2-ones (179). Lack of stereoselectivity is a problem, but facile epimerization is possible because of the electron withdrawing 3-substituent (78T1731). 

A third approach to 3-amino-/3-lactams is by Curtius rearrangement of the corresponding acyl azides. These are readily prepared from r-butyl carbazides, available via photochemical ring contraction of 3-diazopyrrolidine-2,4-diones in the presence of f-butyl carbazate (c/. Section 5.09.3.3.2). Thus treatment of (201) with trifluoroacetic acid followed by diazotiz-ation gives the acyl azide (202) which, in thermolysis in benzene and subsequent interception of the resulting isocyanate with r-butanol, yields the protected 3-amino-/3-lactam (203) (73JCS(P1)2907). 

The 2a,7b-dihydrocyclobuta[7>]benzofuran structure, obtained by addition of activated acetylenes to benzofurans (see Houben-Weyl, Vol.E6b, p 148), isomerizes thermally to a 1-benz-oxepin182 184 185 which reacts further to 1-naphthol derivatives at higher temperature182,185 (see Section 1.2 ). Under photochemical conditions, the 1-benzoxepins undergo ring contraction to the starting material.182 184... 

PHOTOCHEMICAL RING CONTRACTION OF 2-ETHOXYPYRROLIN-5-ONES TO CYCLOPROPANONE DERIVAHVES tert-BUTYL N-(l-ETHOXYCYCLOPROPYL)CARBAMATE... 

Photochemical ring contraction of 2-ethoxypyrrolin-5-ones, 59, 132 Photocycloaddition reactions, 57, 116 Photolysis, apparatus for, 55, 17 59, 132, 195... 

Nitrones derived from 2-azabicyclo[5.3.0]decane give quinolizidine compounds by photochemical Beckmann rearrangement which implies simultaneous ring expansion and ring contraction reactions. Intramolecular Schmidt reactions in 2(4-azidobutyl)-cyclopentanones also give quinolizidinone derivatives by ring expansion. Examples of both types of reactions are given in Sections 12.01.11.1 and 12.01.11.3, respectively. 

The photoelimination of nitrogen from diazo compounds provides a simple and versatile route for the generation of carbenes, and in certain instances, insertion reactions of carbenes can be employed in the synthesis of heterocycles. Carbenes are believed to be involved at least in part in the photochemically induced conversion of N,N-diethyldiazoacetamide (439) into the y-lactam 440 and the /Mactam 441,365 and a similar approach has been successfully employed in the synthesis of a carbapen-2-em366 and of 7-methylcephalosporin analogues.367 Carbene insertion of a different type has been observed on irradiation of the 6-anilino-5-diazouracils 442 to give the indolo[2,3-d]pyrimidines 443.368 Ring contractions in heterocycles... 

The photochemical Wolff rearrangement represents a generally useful ring contraction technique. 

Other approaches to alkylidenecycloproparenes have been attempted without success. Aromatization of appropriate alkylidenecyclopropanes or their precursors could not be realized, and flash vacuum pyrolysis of methylene phthalide and 3-methylene-2-coumaranone afforded rearrangement products rather than alkylidenecycloproparenes via extrusion of 002. The photochemical or thermal decomposition of the sodium salt of benzocyclobutenone p-toluenesulfonyl hydrazone led to products derived from dimerization of the intermediate benzocy-clobutenylidene, or from its reaction with the solvent, but no ring-contracted products were observed. When the adduct of methylene-l,6-methano[10]annulene to dicyanoacetylene (249) was subjected to Alder-Rickert cleavage, phenylacety-lene (250) was formed, which derives reasonably from the parent 234. ... 

Compounds 217 and 218, which are the hydrated forms of 118 and 182, give the same products when treated with 8% NaOH (Equation 16 see Equation 3 for comparison) <1992T4545>. The ring expansion of 201 (Equation 17) <2007S225> and photochemical ring contractions of benzothiazines (Scheme 23) <1994TL3365> have also been reported. 

It should be noted, however, that the 1,3-dipolar cycloaddition chemistry of diazo compounds has been used much less frequently for the synthesis of natural products than that of other 1,3-dipoles. On the other hand, several recent syntheses of complex molecules using diazo substrates have utilized asymmetric induction in the cycloaddition step coupled with some known diazo transformation, such as the photochemical ring contraction of A -pyrazolines into cyclopropanes. This latter process often occurs with high retention of stereochemistry. Another useful transformation involves the conversion of A -pyrazolines into 1,3-diamines by reductive ring-opening. These and other results show that the 1,3-dipolar cycloaddition chemistry of diazo compounds can be extremely useful for stereoselective target-oriented syntheses and presumably we will see more applications of this type in the near future. 

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