Compounds, optically active Ethylene

N-labeled 23,413 suppl. 24 -, S-labeled 12,671 suppl. 24 low boiling s. Low-boiling compounds optically active s. Stereoisomers organometallic s. Organo-metallic compounds -, unsatnared s. Acetylene..., Ethylene... 

Analogous to the use of chiral acetals one can employ chiral N,O-acetals, accessible from a, -unsatu-rated aldehydes and certain chiral amino alcohols, to prepare optically active -substituted aldehydes via subsequent Sn2 addition and hydrolysis. However, the situation is more complicated in this case, since the N,0-acetal center constitutes a new stereogenic center which has to be selectively established. The addition of organocopper compounds to a, -ethylenic oxazolidine derivatives prepared from unsaturated aldehydes and ephedrine was studied.70-78 The (diastereo) selectivities were rather low (<50% ee after hydrolysis) in most cases, the highest value being 80% ee in a single case.73 There is a strong solvent effect in these reactions, e.g. in the addition of lithium dimethylcuprate to the ( )-cinnamaldehyde-derived oxazolidine (70 Scheme 28) 73 the (fl)-aldehyde (71) is formed preferentially in polar solvents, while the (S)-enantiomer [ent-71) is the major product in nonpolar solvents like hexane. This approach was utilized in the preparation of citronellal (80% ee) from crotonaldehyde (40% overall yield).78... 

The dithiocine tetraoxide derived from cyclocondensation of binaphthodithiol with dichloroethylene and oxidation (eq 8) is a chiral version of the bis(phenylsulfonyl)ethylenes. These compounds are useful acetylene equivalents in cycloaddition reactions (see l,2-Bis(phenylsulfonyl)ethylene). Indeed, a chiral acetylene equivalent allows the preparation of optically active hydrocarbons which would be difficult to prepare by classical methods. The dithiocine tetroxide reacts with nonsymmetric dienes to give a single crystalline diastereomeric adduct in most cases. Adducts (1) and (2) were obtained from acyclic and cyclic dienes. 

Photocycloadditions of ethylene with chiral lactam 1.92 (R1 = R" = Me, R = /-Pr) take place selectively on the exo (convex) face of the a,(3-unsaturated system. After acid treatment and epimerization of the liberated methyl ketone, Meyers and coworkers [327] obtained ketone 9.19 with an excellent enantiomeric excess (Figure 9.8). This compound is a precursor of optically active grandisol. Photocycloaddition of chiral fiimarate 9.20 with frans-stilbene is also highly stereoselective [1533] (Figure 9.8). 

Dehydration of the optically active relay (178) derived from enmein (62) gave a 1 2 mixture of 5,6-ene and 6,7-ene. Separation could be achieved by means of the ethylene acetal (187), whose ozonolysis product was subjected to successive Jones oxidation, methylation, Wittig reaction, and treatment with dilute hydrochloric acid to afford the 3-on-16-ol derivative (188). Bromination of (188) followed by dehydrobromination and subsequent dehydration afforded (189). The purified compound (189), after conversion into the acetal, was hydrolyzed to carboxylic acid (190), which was transformed into the desired lactone (191) by treatment with boron trifluoride etherate. The reaction produced a single product uncontaminated by the C-1 epimer, because of easy formation of a favored transition state which satisfied the stereoelectronic requirements. 

FVom the fact that optical activity is never observed in substituted ethylenes (neglecting cases where the substituents are optically active themselves) we conclude that all these compounds are planar, i.e. the positions in the middle with the doubly bonded C-atoms are in a plane with the atoms to which they are immediately bound. 

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