Ring fragmentation

All of these effects can be accounted for by extensions of an additivity scheme, when special increments are attributed to monocyclic structures and the combination of two ring systems having one, two, or three atoms in common [23]. Combination of a table containing values for these ring fragments with an algorithm for the determination of the smallest set of smallest rings (SSSR) [24] (see also Section 2.5.1) allows such a procedure to be performed automatically. 

Silyl ethers serve as preeursors of nucleophiles and liberate a nucleophilic alkoxide by desilylation with a chloride anion generated from CCI4 under the reaction conditions described before[124]. Rapid intramolecular stereoselective reaction of an alcohol with a vinyloxirane has been observed in dichloro-methane when an alkoxide is generated by desilylation of the silyl ether 340 with TBAF. The cis- and tru/u-pyranopyran systems 341 and 342 can be prepared selectively from the trans- and c/.y-epoxides 340, respectively. The reaction is applicable to the preparation of 1,2-diol systems[209]. The method is useful for the enantioselective synthesis of the AB ring fragment of gambier-toxin[210]. Similarly, tributyltin alkoxides as nucleophiles are used for the preparation of allyl alkyl ethers[211]. 

Construction of the A ring fragment starts with epoxidation of chiral d-carvone (114) to afford epoxide 115. 

Condensation of the ylide from 144 with the A ring fragment (as its TMS derivative) gives, after removal of the protecting groups, the vitamin D metabolite cacitriol (145). ... 

The outcomes of intramolecular cyclizations of hydroxy vinylepoxides in more complicated systems can be difficult to predict. In a study of the synthesis of the JKLM ring fragment of dguatoxin, epoxide 44 was prepared and subjected to acid-mediated cydization conditions (Scheme 9.24) [114]. Somewhat surprisingly, the expected oxepane 45 was not formed, but instead a mixture of tetrahydropyran 46 and tetrahydrofuran 47 was obtained, both compounds products of attack of the C6 and C5 benzyl ether oxygens, respectively, on the allylic oxirane position (C3). Repetition of the reaction with dimsylpotassium gave a low yield of the desired 45 along with considerable amounts of tetrahydropyran 48. 

A saturated ring with an aliphatic side chain favors cleavage at the bond connecting the ring to the rest of the molecule. Compounds containing cyclohexyl rings fragment at m/z 83, 82, and 81, corre-... 

Clark and coworkers utilized enyne RCM for constructing the AB ring fragment of the manzamine alkaloids (Scheme 83) [177]. Exposing metathesis precursor 423 and ethylene gas to catalyst A provided bicycle 424 in near quantitative yield. Regioselective hydroboration of the vinyl group in 424, followed... 

High pressure cycloaddition of cytraconic anhydride (140) with 2-substituted furans 142 afforded, cxo-diastereoselectively but unregioselectively, bicyclic cycloadducts 143 and 144 that have been used in straightforward routes to CD-ring fragment of paclitaxel [55] (Scheme 5.22). The cycloadducts were then... 

In 1990, Baumeister et al. [127] described the crystal and molecular structure of 4-ethoxy-3 -(4-ethoxyphenyliminomethyl)-4 -(4-methoxy-benzoy-loxy)azobenzene. The molecules have a bifurcated shape. The phenyliminom-ethyl branch is bent markedly from the nearly linear three ring fragment, but is almost coplanar with the azobenzene moiety. They found that the molecular conformation is affected by an intramolecular interaction of the carboxylic and azomethine groups. The crystal packing was described in terms of a sheet structure with interdigitating rows of molecules. 

The synthesis of the BC-ring fragment 26 begins with the enantioselective aldol reaction of aldehyde 22 and chiral boron enolate 23 derived from (—)-DIP-Cl as described by Paterson and Brown [42]. The aldol adduct 24 is converted to the... 

---