Oxabicyclic systems

Desymmetrizations by C—H insertion using diazo decomposition strategies is well established. Chiu and coworkers have recently applied this strategy to the oxabicyclic system 195. A ferf-leucine-derived Rh(II) catalyst provided the best results [Eq. (10.58a)]. Wardrop and coworkers have applied a similar strategy en route to sordidin unfortunately, both yield and ee proved to be suboptimal [Eq. (10.58b)] ... 

An efficient strategy for the synthesis of /raor-fused oxabicyclic systems involving this annulation followed by a Ramberg-Backlund olefination (Equation 47) as the key step is used for the ring contraction of 116 to give the oxepane 117 in 41% yield, in which the intermediates formed on a-chlorination and sulfone formation steps were used without purification <1996TL2865>. 

Ring-opening reactions in oxabicyclic systems as a route to cyclic and acyclic compounds with several stereocenters 93SL177. 

Syntheses of rifamycin S and ionomycin using ring opening of oxabicyclic systems 92AC1873. 

Mascarenas appUed a strategy related to type c with an alkoxy radical to the chemistry of oxabicyclic systems (see 89 90). He uncovered a very... 

This reaction is not useful for synthesis of the [2.2.1] oxabicyclic system attempts to prepare this system by alternate approaches have been unsuccessful. Again, a more detailed report of this chemistry will be published elsewhere. 

The aim of this section is to give an overview of the general approaches that have been employed. A comprehensive review of all of the methods used to date for the synthesis of oxabicyclic compounds is beyond the scope of the review. Instead, a focus on the preparation of those oxabicyclic systems for which ring opening reactions have been developed is described. The more recent achievements in this area will be emphasized. 

Sammes synthesized /5-bulnesene by employing a sodium reduction of chloroether 248 to effect the ring opening of the bridging C-0 bond in a [3.2.1] oxabicyclic system, Eq. 152 [56]. 

One problem associated with the use of DIBAL-H under such vigorous conditions (i. e. refluxing hexanes) is the appearance of an over-reduced side product, 264, which was difficult to separate from the desired cycloheptenol 263, Eq. 165. The presence of this product indicates a lack of chemoselectivity associated with DIBAL-H for some types of cyclic alkenes versus an olefin in an oxabicyclic system. 

These reactions are useful as routes to aza- and oxabicyclic systems. 

Rhodium catalysts have also been used with increasing frequency for the allylic etherification of aliphatic alcohols. The chiral 7r-allylrhodium complexes generated from asymmetric ring-opening (ARO) reactions have been shown to react with both aromatic and aliphatic alcohols (Equation (46)).185-188 Mechanistic studies have shown that the reaction proceeds by an oxidative addition of Rh(i) into the oxabicyclic alkene system with retention of configuration, as directed by coordination of the oxygen atom, and subsequent SN2 addition of the oxygen nucleophile. 

Preliminary results on the enantioselective formation of sulfur and nitrogen mediumsized heterocycles by base-induced ring opening of hetero-oxabicyclic [3.2.1] and [3.3.1] systems have been reported.91 The reaction involves a deprotonation-C—O bond elimination sequence. The kinetics and mechanism of gas-phase unimolecular elimination reactions of some substituted aminoazoles have been studied as an aid to heterocycle synthesis.92... 

The oxygen bridge in oxabicyclic compounds is an electron pair donor that can stabilize a-carbocations. This characteristic renders oxabicyclic substrates more susceptible to carbocationic skeletal rearrangements resulting in the cleavage of the carbon framework. One such reaction was exploited by Sammes for the synthesis of ( )-cryptofauronol, in which treatment of 145 with Lewis acid induces rearrangement to a decalin ring system, Eq. 100 [57]. 

An intramolecular nucleophilic addition to construct fused bicyclic systems was recently developed by Lautens and Kumanovic [220]. The iodopropyl-sub-stituted oxabicyclic substrate 295 a underwent transmetallation with f-BuLi... 

Silylcupration of the oxabicyclic ketone (8) takes a different course, in that the bridged oxygen system is not cleaved. Instead, the initial exo adduct (9) produces a cyclobutanol product (10) exclusively (eq 11). ... 

Oxabicyclic substrates are attractive precursors to tetrahydrofurans as a consequence of the ease with which they are synthesized and that they lead to the exclusive generation of 2,5-c -tetrahydrofurans. Included in this section is one example of an enzymatic approach to tetrahydrofurans from an oxabicyclic ring system and several examples of ring-opening metathesis approaches to tetrahydrofurans. 

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