7-Oxanorbomene derivative

Hoping to improve or to shorten our total synthesis of L-daunosamine, we explored the following reactions with 112 and 113, the adducts of the racemic 7-oxanorbomene derivative 47 + 48 to PhSeCl and PhSCl, respectively. Saponification of 112 and 113, followed by treatment with formalin, afforded ketones 114 and 115, respectively. Treatment of with tributyltin hydride in toluene/benzene (AIBN 1 - 2%, 80 °C) gave the key intermediate ( )-I03 in 69 % yield. Under the same conditions, 115 was reduced to ( )-I03 in 40 - 45 % yield. Raney nickel reduction of 115 afforded ( )-103 in 50 % yield together with 40 % of ( )-7-oxabicyclo[2.2.1]heptan-2-one. However, we found the multistep procedure 32 100 101 102 103 easier to scale up. Several intermediates in our synthesis do not have to be isolated. For instance, transfomation of 32 into 102 can be carried out in the same pot in 94 % yield. 

In addition to those listed in Tables 1 and 2, many more active catalyst systems have been reported, including W-carbyne complexes [68], and metallacycles such as Ti or Ta cyclobutanes [8, 69], which are initiators for ROMP. The ROMP of functionalized monomers such as 7-oxanorbomene derivatives can also be performed successfully using certain Ru initiators, affording polymers with a high molecular weight (eq. (18)) [70]. Moreover, in water alone, polymerization proceeds very rapidly in nearly quantitative yields in the presence of Ru catalysts under an atmosphere of air [71] (cf. Section 3.3.10.1). 

The observation that these polymerizations proceeded in alcoholic solution led to the discovery that selected Ru and Ru " complexes catalyze the ROMP of these 7-oxanorbomene derivatives in water alone to provide quantitative yields of the desired ring-opened polymer (eq. (6)) [29]. 

Upon UV irradiation, complexes of the type [(j -arenei)Ru( / -arene2)] and [Ru(NCR)6] are transformed into active catalysts for the ROMP of norbomene and 7-oxanorbomene derivatives (Karlen 1995). 

Norbomene (Mi) has been copolymerized with a number of its derivatives and with 7-oxanorbomene derivatives (M2), using RuCls and other catalysts. The monomers are of comparable reactivity (ri = 0.5-2.6) and the behavioiu close to ideal (rir2 = 0.9-2.2) (Monakov 1995 Tlenkopatchiev 1995c). 

Novak, B. M. Grubbs, R. H. Catalytic organometaUic chemistry in water The aqueous ring-opening metathesis polymerization of 7-oxanorbomene derivatives. J. Am. Chem. Soc. 1988, 110, 7542-7543. 

These reactions can also be strongly biased by the choice of monomer and catalyst. The polymerization of substituted, polar 7-oxanorbomene derivatives with apolar tydoalkenes by GoughUn and Ilker afforded up to 98% alternating monomer utilizing G1 while the more active catalyst G2 yielded at best 85% of the desired pattern. Similarly, use of GOD or cydooctene enable > 90% alternation while this value plummeted to 40% when norbomene was used as the nonpolar monomer. ... 

Figure 16.15. ROMP of oxanorbomene derivatives in water with a ruthenium cataly st...

Enzyme catalyzied polymerization Oxanorbomene- derivative (BMM-7-ON) Ru-based catalyst P(BMM-7-ON) Synperonic FI 27 40-60 38... 

The reaction of the 2,5-bis-trifluoromethyl-l,3,4-oxadiazole 212 with oxanorbomene derivatives has been recently re-evaluated for its stereoselectivity aspects, through a combination of experimental and computational studies [105], In particular, the theoretical model was able to explain the origin of stereoselectivity towards the bent product 214 caused by repulsive lone pair interactions between oxygen bridges in the transition state of the 1,3-dipolar addition (Scheme 54). 

Scheme 54 Stereoselectivity towards the bent product 214 in the reaction of the 2,5-bis-trifluoro-methyl-l,3,4-oxadiazole 212 with oxanorbomene derivatives...

A benefit of functionalized homopolymers is the ability to perform postpolymerization modifications. A norbomene bearing a methacroyl isocyanate efficiently underwent homopolymerization via ROMP to yield a cross-linked homopolymer, which upon ligand exchange with poly(methyl methacrylate) afforded a material with enhanced thermal stability over the original poly(methyl methacrylate). The preparation of an oxanorbomene derivative bearing a 1,2-bis (3-thienyl)cyclopentene photochrome followed by ROMP accessed a polymer capable of the same reversible photoisomerization as the monomer. 

Tacticities have been determined for polymers of various derivatives of norbomene, norbomadiene and 7-oxanorbomene. Three methods have been used (i) by polymerizing single enantiomers of 5-, or 5,5-, or e rfo,exo-5,6-disubstituted derivatives, and determining whether the polymers have an HT (m) or HH/TT (r) stmcture, or both (m and r) (ii) by polymerizing 5,6-disubstituted derivatives in which the substituents contain a chiral centre of a single handedness, and... 

Studies of this kind fall broadly into two groups as indicated in Table 15.1. Those labelled P are mainly concerned with the effect of small quantities of acyclic olefin M2 on the MW, yield, and cis content of the polymer produced from the cyclic olefin Mi in this case the ratio [M2]/[Mi] is usually 0-0.05. Those labelled T involve the use of much higher proportions of M2 (with [M2]/[Mi] 0.5-2), with the object of producing telomers with well-defined end-groups, in some cases for synthetic purposes. For example, highly substituted cyclopentane and tetrahydro-fiiran derivatives can be readily made by the cross-metathesis of substituted norbomenes or 7-oxanorbomenes with hex-3-ene or l,4-dimethoxybut-2-ene catalyzed by ruthenium carbene complexes (Schneider, M.F. 1996). If the acyclic olefin is unsymmetrical and represented by Q Q, where Q and are the alkylidene moieties, three series of telomers may be produced, Q (Mi) Q, Q (Mi) Q, Q (Mi) Q, where ti is the number of Mi units. The lowest members of the series (n= 1) are dienes, and it is sometimes possible to deteet, separate, and identify the cc, ct, and tt isomers. 

Polymers of 7-oxanorbomene and 7-oxanorbomadiene derivatives can be degraded to telechelic oligomers using (Me3SiCH2CH=)2 with Mo(=CHCMe2Ph) (=NAr)[OCMe2CF3]2 as initiator (Viswanathan 1994). 

PLA-functionalized polyoxanorbomenes with one or two exo-FLA chains as well as two endo-, exo-chains were prepared using Sn(Oct)2 as a catalyst in the presence of mono- or dialcohol derivatives of oxanorbomenes [73]. These macromonomers are then subjected to ring-opening metathesis polymerization (ROMP) to yield graft copolymers (Figure 4.8). 

Figure 25 Synthesis of 7-oxanorbomene macromonomer derived from two (G-1) den-drons followed by ROMP polymerization into a linear dendritic hybrid architecture. (Courtesy of Macromolecules 1997,30(19) 5786. Copyright 1997 American Chemical Society.)...

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