Oxanorbornenes

A subsequent publication by Blechert and co-workers demonstrated that the molybdenum alkylidene 3 and the ruthenium benzylidene 17 were also active catalysts for ring-opening cross-metathesis reactions [50]. Norbornene and 7-oxanorbornene derivatives underwent selective ring-opening cross-metathesis with a variety of terminal acyclic alkenes including acrylonitrile, an allylsilane, an allyl stannane and allyl cyanide (for example Eq. 34). 

Being a diastereomer of 450 with respect to the configuration of the sulfur atom, 458 was liberated from the triflate 457 by ethyl diisopropylamine and trapped by furan (Scheme 6.93). The resulting [4+ 2]-cycloadduct 459 was isolated in 62% yield and is a diastereomer of 451 [155, 171b], Typical for virtually all furan adducts of six-membered cyclic allenes, 451 and 459 display the mdo-configuration with respect to the 7-oxanorbornene skeleton. 

Synthesis of Carbohydrates and Derivatives from 7-Oxanorbornenes ( Naked Sugars )... 

Alkenes have also been prepared by retro-Diels-Alder reaction of resin-bound cyclohexenes. Figure 3.40 depicts an interesting example of this strategy, in which the retro-Diels-Alder reaction is induced by conversion of a thermally stable, resin-bound 7-oxanorbornadiene into a thermally unstable 7-oxanorbornene [795]. 

Despite being forbidden by the Baldwin rules (5-mdo-trig ring opening see Section 9.2), cyclohexadienoic acid derivatives such as that required for this synthesis can be prepared by base-induced ring scission of 7-oxanorbornene derivatives, presumably because of the high strain-energy of norbornenes. The required 7-oxanorbornene, in turn, should be readily accessible from furan and an acrylate via the... 

Diels-Alder reaction. With the aid of an enantiomerically pure Lewis acid this Diels-Alder reaction yields a highly enantiomerically enriched 7-oxanorbornene, so that the remaining steps of this elegant synthesis only need to proceed diastereo-selectively and without racemization. 

The ring-opening metathesis polymerization (ROMP) of 7-oxanorbornene derivatives initiated by Ru(H20)6(4-toluenesulfonyl)2 in aqueous media was reported by Novak and Grubbs [45] (Eq. 20). Compared with the same reaction carried out in organic solvent, the initiation time was greatly decreased. After the polymerization, the aqueous catalyst solution was not only reused but also became more active in subsequent polymerizations. 

The diastereoselective and enantioselective [2+2] cycloaddition of a 7-oxanorbornene with a chiral alkynyl acyl sultam was effected by using a ruthenium catalyst to provide the . 

Different dendritic side groups were linked to 7-oxanorbornene via an ester group (XXXII-XXXVI) (Fig. 20) [84-86]. The monomers by themselves also assembled into a supramolecular arrangement of spherical or columnar shapes. 

Beside classical SCLCPs, attaching dendritic side chains to poly(norborn-enes) and poly(7-oxanorbornenes) leads to highly-ordered columnar mesophases (Sect. 2.5). In these polymers, the dendritic side chains force the polymer to adopt a rod-like structure. 

Furan adds to vinylene carbonate to produce an exolendo mixture of Diels-Alder adducts [165]. Double hydroxylation of the 7-oxanorbornene double bond is highly exo face selective (Scheme 13.88). The diol thus obtained is protected as an acetonide. Saponification of the carbonate liberates a mixture of diols that is oxidized into m 5 6>-l,5-anhydroallaric acid derivative 316. Treatment of 316 with AC2O generates the anhydride 317. Subsequent reaction of 317 with methanol gives racemic 318 that can be resolved by fractional crystallization with brucine or by chromatographic separation of the (7 )-l-((3-naphthyl)ethylamides. The individual isomers of 318 each react with ClCOOEt and Me3SiN3 in situ to provide enantiomerically pure d- and L-riboside derivatives [166]. 

The ROMP of 7-oxanorbornenes substituted with glucose or mannose moieties bound via C- or O-glycosidic linkages afforded neoglycopolymers, such as 19 [120, 121]. 

The development of living ring-opening metathesis polymerization (ROMP cf. Section 6.10) catalysts was greatly influenced by the initial use of hydrated late transition metal salts. In the aqueous polymerization of 7-oxanorbornene derivatives, RuC13 and [Ru(H20)6]2+ were found to produce polymer with the latter having a smaller induction period [68], A key intermediate and product in the reaction is the alkene adduct, [Ru(H20)5(alkene)]2+ with the use of RuC13 and... 

The Group VIII complexes discussed above are generally limited to the ROMP of functionalized norbornenes and 7-oxanorbornenes (cf. also Section 6.12.6). These... 

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