Oxygen isopropylidene derivatives

The assignment of proton chemical shifts is also supported by carbon-13 chemical shifts obtained for the 5,6-isopropylidene derivatives of DHA, iherythro-DHA and 6-bromo-6-deoxy-L-DHA. All the dehydro compounds were prepared by oxidation using oxygen over charcoal in 95% ethanol. The preparation of the isopropylidene derivatives follows. 

Taxanes.—The cage-like structure (117) of the taxinines possessing an 1 l-en-13-one system allows the formation of a transannular C-3—C-11 bond on irradiation. However, the C-13 carbonyl is held away from C-3 so that the oxygen atom cannot participate in an intramolecular hydrogen-abstraction process. Thus irradiation of the diacetate (118) afforded (119), which was related by acetylation to taxinine L, a minor constituent of T. cuspidata. When the C-9 and C-10 hydroxy-groups were converted into their isopropylidene derivative, the central ring was sufficiently deformed to increase the separation between C-3 and C-11. A different photochemical reaction then occurred and a cyclopropyl ketone (120) was obtained. 

A range of polysulphones has been prepared with a variety of bis-phenols other than bis-phenol A. As might be expected from the discussion in Chapter 4 and from experience with the range of polycarbonates (Chapter 20), replacement of the isopropylidene link with a methylene, sulphide or oxygen link depresses the Tg whilst —C(CgH5)2— and sulphone links raise it. The bis-phenol derived from norcamphor leads to a polysulphone with a Tg of 250°C (195°C for a Udel-type polymer). 

The JV-benzyl aziridines 69, 92-95 which differ only in the nature of the norbomane bridge (CH2, spirocyclopropyl, isopropylidene, oxygen, substituted nitrogen) (Scheme 14), have been prepared and reacted with each of the corresponding benzonorbomadienes 36-40 from which they were derived. All 25 reactions were conducted to produce 14 of the possible 15 different... 

One would also think that saccharides are more attractive starting raw materials for direct preparation of epoxy resins than hydrocarbons since they already possess oxygen atoms needed for the epoxy rings. Such diepoxy derivatives of hexitols are known compounds, for instance 1,2 5,6-dianhydro-3,4-0-isopropylidene-D-mannitol (XIX). Their preparation, however is difficult and requires 15 to 18 steps which proceed in very low overall yield (17-19). 

In the synthesis of D-tagatose from the more common D-fructose, 1-O-benzoyl-2,3-O-isopropylidene-P-D-fructopyranose afforded two products identified as l-O-benzoyl-5-0-benzyl-2,3-0-isopropylidene-P-D-fructopyranose (97%) and its 4-O-benzyl isomer (2.8%). The skew-boat 6S4(D) conformation with an oxygen atom at C-5 adopting a quasi-equatorial position is responsible for the unexpected regioselectivity observed [136]. Conformational equilibria may also be a reason for the non-exclusive, though preferential substitution at 0-3 of benzyl 4-0-benzyl-6-deoxy-a-L-talopyranoside (9) [142]. Even in this case, however, no tri-O-benzyl derivative was formed and no starting material 9 remained, the total isolated yield of 10 and 11 being 87%. 

When reactions are performed on tributylstannyl ethers and dibutylstan-nylene acetals of hexopyranoside derivatives that have more oxygen atoms, including the primary one, unprotected, markedly different results are obtained, as shown in Figs. 24 and 25. Dibutylstannylene acetals favor reaction on the position in the 1,2-diol unit that is adjacent to an axial substituent, whereas tributyltin ethers prefer to react at the primary centers. However, this pattern was not observed for reactions of the tributylstannyl ether of l,2-(l-methoxyethylidene)-j3-D-mannopyranose, as shown in Fig. 26.122 Methylation of this compound through the tributylstannyl ether in toluene in the presence of added tetrabutylammonium bromide also gave substitution on 0-3 predominantly.123 These results may arise from distortion of the chair conformation by the fused isopropylidene acetal. 

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