Lactones conformations

Unsaturated -lactone 34 adopts a well-defined conformation and provides a suitable platform for the introduction of the stereogenic center at C-24 (monensin numbering). Catalytic hydrogenation of the carbon-carbon double bond in 34 takes place preferentially from the less hindered side of the molecule and provides an 8 1 mixture of stereoisomers in favor of 35 (100% yield). Cleavage of -lactone 35 with concentrated hydriodic acid at 130°C, followed by treatment of the resultant iodide 36 with triphenylphosphine, completes the synthesis of intermediate 19. 

Jorgensen et al. [84] studied how solvent effects could influence the course of Diels-Alder reactions catalyzed by copper(II)-bisoxazoline. They assumed that the use of polar solvents (generally nitroalkanes) improved the activity and selectivity of the cationic copper-Lewis acid used in the hetero Diels-Alder reaction of alkylglyoxylates with dienes (Scheme 31, reaction 1). The explanation, close to that given by Evans regarding the crucial role of the counterion, is a stabilization of the dissociated ion, leading to a more defined complex conformation. They also used this reaction for the synthesis of a precursor for highly valuable sesquiterpene lactones with an enantiomeric excess superior to 99%. 

Cyclic a-methylene ketones and lactones, in which the syn conformation is enforced, give predominantly exo adducts.14... 

P2j Z = 2 Dx = 1.363 R = 0.051 for 982 intensities. The lactone has the3T4 conformation, withQ = 13 pm, (p = 337°, andaC-2-C-3-C-4-O torsion-angle of — 120. Both the equatorial and the axial acetoxyl groups have their planes approximately normal to that of the lactone ring. 

P2j Z = 2 D, = 1.24 R = 0.05 for 2,421 intensities. This molecule is a cardenolide / -D-glucopyranoside. The pyranoside conformation is 4C4, with Q = 58 pm, 0 = 6°. The terminal lactone ring is planar. The conformation was compared with that of digitoxigenin by means of molecular-mechanics calculations. 

Enol lactones are assumed to form from iV-methylisoquinolinium salts as a result of a Hofmann-type degradation process. This P elimination is a highly stereospecific reaction in which Z isomers are produced from precursors of erythro configuration and isomers from threo diastereomers(5,97). This fact seems to suggest that syn rather than the more usual anti elimination takes place. Examination of models indicates, however, that there is a preferred conformation in which the C-8 hydrogen is in the syn and coplanar position to the quaternary nitrogen. This hypothesis was proved correct in experiments carried out in vitro (5,14,15,91-94). 

W=X is N=N or C=N, never if it is C=C. The ring stability of 3-furyl-carbenes conforms with this rule. Ring opening is again the main reaction in a biradical which is generated by extruding carbon dioxide from a lactone at 675°C and which then collapses to an acetylenic ketone.277... 

In contrast to the D-glucopyranose-D-glucofuranose equilibrium, in which the former preponderates, compound 4, because of its more favorable conformation, is thermodynamically more stable than D-glucopyranurono-6,3-lactone (9). 

V. Structure-Reactivity Correlations. Conformation of d-Glucofuranosidurono-6,3-lactones... 

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