Carbocycles macrocyclic

In the field of synthesis of natural macromolecular carbocycles, macrocyclization through formation of a carbon-carbon bond has been intensively investigated. We will give just a few examples the alkylation of carbanions (Fig. 1 la, I lb) the addition of allylstannane to aldehyde (Fig. 11c) and the intramolecular Wittig reaction (Fig. 1 Id). ... 

Sigmatropic Wittig rearrangement of macrocyclic allyl propargyl ethers to carbocycles 91TA1. 

Trost and coworkers [116] found that palladium-catalyzed intramolecular allylic alkylation to a-sulfonyl ketones is a good means of performing macrocyclization. This reaction involves the intermediacy of a tt-allylpalladium complex as an enolonium equivalent to initiate cyclization. For instance, this method was used in the synthesis of the cytochalasin ( — )-aspochalasin B (201) [117]. As shown in Scheme 66, cyclization of the linear precursor 199 using 10 mol% (Ph3P)4Pd in the presence of 10 mol% DPPP in THF created the 11-membered carbocycle 200 as a single diastereoisomer in 49% yield. 

Takahashi and coworkers have employed intramolecular reactions of anions of unsaturated protected cyanohydrins to produce unsaturated carbocyclic systems containing 10-, 14- and 16-membered rings which are convertible into natural products. These reactions occur regiospecifically at the a-position and usually do not involve ( ) — ( isomerization of P,7-double bonds. The preparation of 2-cyclopen-tadecenone (155), which is convertible into the macrocyclic perfumes muscone and exaltone, is illustrative of this methodology (Scheme 76). 

BLOMQUIST Macrocycles Synthesis Synthesis of large ring carbocycles by cyclization of bifunctional ketenes. 

A whole series of cytochalasans was identified from the same raw material, including three new ones penochalasins A (131 a colorless needle extraction yield 0.7 mg/ml), B (132 a colorless powder 0.4 mg/ml) and C (133 a colorless powder 0.2 mg/ml). The other two compounds isolated were already known chaetoglobosins A (134 0.5 mg/ml) and F (137 0.4 mg/ml). The stereostructure was determined on the basis of NMR studies and chemical transformations. Cytochalasins are highly substituted perhydroisoindol-l-one fused with a macrocyclic (a carbocycle, lactone or carbonate). Penochalasins constitute a novel class of... 

Finally, cytochalasins possessing a macrocyclic lactone system, as represented by cytochalasin B, are obtained from carbocyclic precursors by a Baeyer-Vil-liger-type oxidation. In the case of a cyclic carbonate group, such as in cytochalasin E and K, the carbonate moiety might result from a second Baeyer-Villiger-type insertion of an oxygen atom into a lactone ring. 

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