Diazo ring contraction

A-Dinor-5a-cholestan-l-one has been prepared by Cava and Vogt by a multistep sequence involving, as the final ring contraction reaction, photolysis of 2-diazo-A-nor-5a-cholestan-1 -one. 

With cyclic a-diazo ketones, e.g. a-diazo cyclohexanone 9, the rearrangement results in a ring contraction by one carbon " ... 

In the context of 12, the diazo keto function and the thiolactam are in proximity. This circumstance would seem to favor any process leading to the union of these two groupings. It is conceivable that decomposition of the diazo function in 12 with rhodium(n) acetate would furnish a transitory electron-deficient carbene which would be rapidly intercepted by the proximal thiolactam sulfur atom (see 20, Scheme 4). After spontaneous ring contraction of the... 

Ring contraction of cyclic diazo ketones (Wolff)... 

Photolysis of the first known cyclic a-diazo-p-oxophosphine oxide 49 is unsuccessful with regard to phosphene formation. There is no evidence for a P/C-phenyl shift, which should lead to 51, nor for a P/C-alkyl shift, which would afford 52 via ring contraction, since none of the expected phosphinic esters could be isolated in methanolM). 

Ring contraction by (Si - C) vinyl migration took place when the diazo-silacyclohexadiene 393 was heated in the presence of anhydrous CuS04 388>. The silafulvene 394 was not isolated, but it could be trapped by r-butanol, benzophenone or benzaldehyde to give 395,396a and 396b, respectively. Silane 395 was also obtained... 

Gould and coworkers have extensively studied the biosyntheses of the kinamycins, and this work was recently reviewed [5a]. Feeding studies established that the carbo-cyclic skeletons of the kinamycins are constructed from 10 equivalents of 5-acetyl coenzyme A, and the pathway shown in Scheme 3.4 was proposed. The pathway begins with formation of the natural product dehydrorabelomycin (29). A novel ring contraction then occurs to form the cyclopentadienone 30. Feeding studies with /V-15-ammonium sulfate established that the diazo functional group is then installed... 

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 photolysis of cyclic diazo ketones in hydroxylic solvents leads to ring contracted carboxylic acid derivatives via this ketocarbene -> ketene rearrangement. Examples of such reactions are given in (2.24)239) and (2.25) 240). In this last example a photoequilibrium between the diazo ketone and its valence isomer, a diazirine, has been observed, both products then eliminating nitrogen to afford the cyclobutane carboxylic acid. 

Cyclic diazoalkanes have been used to generate spirocyclopropanes via ring contraction of pyrazolines formed in the initial cycloaddition step. Two novel examples are the transformation of 3-diazo-2-nitromethylenepiperidine (16) into 5-aza-spiro[2.5]octane 17 (45) and the conversion of 1-diazo-2-methylenecyclo-propane (18) into methylene-spiro[2.2]pentane 19 (46) (Scheme 8.6). Related reactions have also been reported for diazocyclopropane (47) and diazospiropen-tane (48). 

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. 

The conversion of 3-amino-4-oxo-l,jc-naphthyridines (117) into the corresponding 3-diazo-4-oxo compounds (118) followed by photolysis results in the formation of azaindoles (119) via nitrogen evolution and ring contraction (56LA(599)233, 60JCS1794). 

It has been demonstrated that x-diazo ketones, e.g. 8, may photochemically equilibrate with their isomeric a-oxodiazirines, e.g. 9, but the Latter upon irradiation also undergo Wolff rearrangement with ring contraction.46... 

Diazo transfer to 1 followed by irradiation in the presence of bis-(trimethylsilyl)amide led to ring contraction with concomitant carbonyl extrusion, to give 7. Dehydration to the nitrile followed selenation then set the stage for a highly diastereoselective ytterbium-catalyzed Diels-Alder reaction, to give, after reduction and protection, the pentacyclic intermediate 2. 

Compounds with a Si=Si or Ge=Ge bond (i.e., disilenes and digermenes) can be isolated when the double bond contains bulky substituents. Only a few cycloaddition reactions with diazo compounds are known, and two reaction modes have been observed. One of the paths leads to the formation of disiliranes 184 (242) and digermiranes 185 (243) (Scheme 8.42), probably by ring contraction of an initially formed [3 + 2] cycloaddition product. The other path involves a 1,1-cycloaddition of the diazoalkane to give disilaaziridine 186 (244) and digermaazir-idine 187 (245). This nitrene-like reactivity is rather uncommon although some intramolecular examples are known (see Section 8.6.1). 

The well-known photochemical transformation of oc-diazo ketones to ring-contracted derivatives also occurs when 3-amino-4-oxo-l,6-naph-thyridine 89 is treated with nitrous acid and the a-diazo ketone 90 is phoio-... 

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