Aldehydes cyclohexylidene—

PREPARATION OF CYANO COMPOUNDS USING ALKYLALUMINUM INTERMEDIATES. I. DIETHYLALUMINUM CYANIDE, 52, 90 PREPARATION OF a,g-UNSATURATED ALDEHYDES via THE WITTIG REACTION CYCLOHEXYLIDENE-... 

These problems were circumvented by protecting the C(4),C(5) diol prior to Wittig olefination step (Figure 3). Thus, treatment of 10b (a mixture of pyranose and furanose anomers prepared by hydrolysis of 8 with aqueous trifluoroacetic acid) with excess EtSH and concentrated HCI (as solvent) at provided dithioacetal 9 in 50% yield, along with 25% of a mixture of thiopyranosides and thiofuranosides that was recycled to 10b in high yield by treatment with HgCIa and CaCOa in aqueous CH3CN. Finally, the diol unit was protected as a cyclohexylidene ketal, and then the thioacetal was hydrolyzed under oxidative conditions to arrive at the key aldehyde intermediate 3. 

There are few reactions of aldehydes and ketones that do not in some way affect the carbonyl function. For this reason, it may be necessary to protect the carbonyl function when it is desirable to avoid reaction at this function. For example, you may plan to synthesize 4-cyclohexylidene-2-butanone by way of a Wittig reaction (Section 16-4A), which would involve the following sequence ... 

Walborsky and coworkers206 have reported the CD of series conjugated cyclohexylidene aldehydes and ketones. These compounds served as additional examples supporting the planar diene rule for CD jt tt transition208. 

The preparation of 1,2-O-cyclohexylidene-a-D-xylofuranose (22) (Expt 5.117) from 1,2-O-cyclohexylidene-a-D-glucofuranose illustrates the use of sodium metaperiodate for the cleavage of carbon-carbon bonds in a-diols (see also Section 4.2.55, p. 454). In this case C-6 is lost as formaldehyde and C-5 is converted into an aldehyde group. This aldehydic product is isolated as a dimer, which is then reduced in methanol solution with sodium borohydride to the xylofuranose derivative (22). 

The nitroaldol condensation with nitromethane (Henry s reaction), followed by Nef decomposition of the resultant nitronate under strongly acidic conditions, has been used to elongate aldehydes. For instance, A-acetyl-D-mannosamine has been converted into A-acetylneuraminic acid applying this method iteratively [69]. Chikashita and coworkers [70] have reported good levels of anti diastereoselectivity better than 99% in an iterative homologation sequence using 2-lithio-l,3-dithiane [71] with 2,3-O-cyclohexylidene-D-glyceraldehyde R)-62. In the case of the BOM-protected tetrose derivative, the addition of 2-lithio-l,3-dithiane was syn selective (synlanti 82 18) (Scheme 13.30). 

Roush s three-carbon-chain elongation method applied to 2,3-0-cyclohexylidene-D-glycer-aldehyde (R)-166 is an efficient approach to D-glucitol derivatives which relies on the syn selective epoxidation of homoallylic alcohol 167 (O Scheme 65) [300,302]. 

O-AIkylidene and -arylidene derivatives of D-gluco-, D-allo-, D-manno-, and D-talo-pyranose are difficult to prepare directly. Lem-ieux and Detert59 prepared these from the 1,2-O-ethoxyethylidene orthoester, and were able to recover quantitative yields of 1,2-O-iso-propylidene, 1,2-O-cyclohexylidene, 1,2-O-cyclopentylidene, and diastereoisomeric 1,2-O-benzylidene acetals of D-glucopyranose. In their procedure, the orthoester is allowed to react, with scrupulous exclusion of moisture, with a ketone or aldehyde in the presence of an acid catalyst. 

---