Ring contraction skeleton

In most cases, the cleavage of a carbon-carbon bond causes rearrangements of the carbon skeleton Ring contraction, ring expansion, and alkyl group migration are observed under different conditions These transformations proceed in most cases in the presence of catalysts at elevated temperatures Examples where only temperature causes rearrangements will be discussed m the next section... 

With cyclic a-halo ketones, e.g. 2-chloro cyclohexanone 6, the Favorskii rearrangement leads to a ring contraction by one carbon atom. This type of reaction has for example found application as a key step in the synthesis of cubane by Eaton and Cole for the construction of the cubic carbon skeleton ... 

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... 

Medium sized carbocycle synthesis Ring contracting [1,2]-Wittig rearrangement Yadav and Ravishankar have demonstrated that the Wittig rearrangement of the cyclic substrate 32 is useful for the construction of the taxane skeleton 33, albeit in low yield (equation 17). ... 

By repeating the Wolff ring contraction twice, the highly strained (94 kcal mol MM144) diasterane skeleton 7 was synthesized.45... 

The structure of factor III, the trimethyl isobacteriochlorin, is worthy of note. Preliminary studies had suggested the structure (81), which was in better accord with a subsequent ring contraction to the corrinoid skeleton involving oxidation at C-20 followed by extrusion of formaldehyde. However, the 13C NMR spectrum of factor III (as its octamethyl ester) enriched biosynthetically with [13CH3]methionine and [5-13C]ALA (Scheme 25) shows both the meso and meso- methyl carbons as doublets, a feature in accord with substitution of... 

The substituents present at the 20 position are characteristic of this structure the methyl group is present in Factor III, an intermediate in the biological conversion of uroporphyrinogen to Vitamin B12- The hydroxyl group represents the hypothetical subsequent biological transformation. Their presence is essential for ring contraction in the absence of the 20-methyl group the conversion to the corrin skeleton does not occur, but there is tautomerizaton to a keto form [69, 79-81],... 

Reductive ring contraction of the 1,2-dithiine skeleton was applied in the formation of the thiophene ring. Thieno[3,4-h]thiophenes 148 were prepared from thiophen-3-yl-substituted [l,3]dithiol-2-ones 144 in good yields [39], The photochemical irradiation of 144 leads to thieno[3,4-c]dithiines 145 via a unique ring cleavage reaction, and subsequent reductive treatment of dithiines 145 proceeds to give 2,3-dihydroth-ieno[3,4-h]thiophenes 146 and 147. Heating of 147 in the presence of further iodomethane leads to the formation of the thieno[ 3,4-/ ] thiophenes 148 (Scheme 27). 

Ylides with substituted carbanions can form vicinal stereocenters upon 2,3-reairangement. In acyclic compounds, little stereocontrol is documented more often than good control. Cyclic ylides, on the other hand, exhibit high stereoselectivities for ring contractions via 2,3-sigmatropic shifts. The oldest and still the most impressive among such reactions is Yoshimoto s transformation of the cephalosporin (162) into the penicillin skeleton (163). Diastereoselectivity follows from an exo preference of the ester group in transition state (164 equation 49). ... 

Uyehara, T., Takehara, N., Ueno, M., Sato, T. Rearrangement approaches to cyclic skeletons. IX. Stereoselective total synthesis of ( )-campherenone based on a ring-contraction of blcyclo[3.2.1]oct-6-en-2-one. Reliable one-step diazo transfer followed by a Wolff rearrangement. Bull. Chem. Soc. Jpn. 1995, 68, 2687-2694. 

A recent investigationinto the biosynthesis of botrydial (452) and dihydrobotry-dial (453) (c/. Vol. 6, p. 95) is described in Chapter 6. The unusual carbon skeleton (455) of these compounds is probably constructed in nature by ring contraction and ring cleavage of an appropriate tricyclic intermediate [cf. (454)] derived from farnesyl pyrophosphate. 

Ring contraction of tetrabromobenzocycloheptenedione provided the l//-cyclo-propa[6]naphthalene skeleton 2. Aromatization to the cycloproparene 3 was effected via reduction of the carbonyl groups to alcohols, conversion to p-toluenesulfonates and subsequent reaction with butyllithium. In contrast, the ring contraction of 5,7-dibromodibenzo[n,c]cy-cloheptadien-6-one 4 gave none of the expected 1 //-cyclopropa[/]phenanthren-l -one. The products of the reaction were derivatives of phenanthroic acid, and the cycloproparenone was probably not a reaction intermediate. ... 

The compounds (Table 4) in the column headed Aldehyde are A-nor- or B-nor-5)5-formyl derivatives (e.g. 574), resulting from ring contraction. The normal steroid skeleton (576) is regenerated if the aldehyde (574) is first reduced, and the tosylate (575) of the resulting neopentyl alcohol then solvolysed. ... 

A synthesis (Scheme 3) of the perhydrocyclopenta[c,d]indoline derivative (66), which represents the skeleton of dendrobine (43 R = H), has been reported.72 Condensation of carvotanacetone (61) with butadiene gave the adduct (62) which was converted into the ring-contracted product (63) using a well-known reaction sequence. The latter was readily transformed to a stereoisomeric mixture of... 

---