Spiro-ether group

Organon have developed Org 31710 and Org 33628 (Figure 7.2). Org 31710 has the same substituent at position 11 but differs at the 17-position, where it contains a spiro-ether group. In the B-ring, it has a 6P-methyl group. Org 33628 has a acetophenone group instead of a dimethylaminophenyl group and is combined with a methylene-furan substituent at position 17 [12]. 

According to the transition state proposed by Nagasawa et al., the catalyst forms a complex with the Z-enolate of Schiffs base through ionic and hydrogen-bonding interactions. In addition, the methyl groups on the spiro ether ring controls the electrophile approach. 

Electrochemical fluorination of tetrahydrofuran carboxylic acids derivatives proceeds with preferential formation of F-tetrahydrofurane (5), however, in case of furans containing carbonyl group in the side chain, an interesting formation of spiro-ethers 7 is observed. F-Oxanes 8 and 9 are prepared in low yield by ECF of the corresponding oxane derivatives (Fig. 9.2). 

The IR spectra in Figure 4 trace the synthetic pathway to the SOCM monomer 3. The entire series is dominated by the strong IR bands characteristic of the spiro orthocarbonate group. IR absorptions associated with the shortened CO4 centr ether bonds occur near 1200 cm while the external ether bands are found near 1050 cm. Intermediates 6 and 7 produce a strong OH band around 3400 cm and 7 also yields peaks due to the oxaspiro-based double bond at 1698 and 880 cm". Addition of the methacrylate group in monomer 3 eliminates the OH band and gives rise to the carbonyl and methacrylate vinyl absorptions at 1720 and 1637 cm", respectively. 

In 2009, the catalytic enantioselective semipinacol rearrangement of 2-oxo allylic alcohols 83 was detailed by Zhang et al., leading to enantioenriched spiro-ethers 84 in a single operation (Scheme 2.24). They found that both phosphoric acids 5b and silver phosphate 5i were optimal catalysts, while the latter probably underwent silver-proton exchange with hydroxyl group of subsnates in the catalytic procedure [35],... 

Giomi s group developed a domino process for the synthesis of spiro tricyclic nitroso acetals using a, 3-unsaturated nitro compounds 4-163 and ethyl vinyl ether to give the nitrone 4-164, which underwent a second 1,3-dipolar cycloaddition with the enol ether (Scheme 4.35) [56]. The diastereomeric cycloadducts formed, 4-165 and 4-166 can be isolated in high yield. However, if R is hydrogen, an elimination process follows to give the acetals 4-167 in 56% yield. 

The allenic ether (204) cyclizes to spiro compounds in the presence of potassium r-butoxide and dicyclohexano-18-crown-6. Acid hydrolysis yields the dihydrofuranone (205). The new carbonyl compound l-oxaspiro[4,4]nonan-4-one can be subjected to another spiroannela-tion sequence. The carbonyl group in (206) has two faces available for addition of a nucleophilic species. Only one product is formed, a cyclopentyl[3]helixane (207). One can in principle continue this reiterative reaction (Scheme 53) (B-81MI31200). 

Selective deprotection of the silyl ether by hydrolysis, oxidation of the free hydroxyl group to the aldehyde 19 and then reaction with carboethoxymethylenetriphenylphosphorane directly afforded the spiro compound 20, which is an interesting bicyclic Michael acceptor. 

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