Asymmetric Diels‐Alder Reactions of Neopentyl‐Ether‐Shielded Acrylates and Allenic Esters: Syntheses of (−)‐Norbornenone and (−)‐β‐Santalene

Abstract

Starting from (+)-or (-)-camphor, the antipodal alcohols 14 and 18, respectively, have been prepared; the corresponding acrylates 15 and 19 underwent TiCl,(i-PrO)2-mediated Diel.vAlder additions to cyclopentadiene to give adducts 20a and 22a respectively, with 95% endo-and 99.2% n-face selectivities. Adduct 22a was converted to enantiomerically pure norbornenone 26. Addition of 1,3-butadiene to acrylate 15 in the presence of TiC1, Introduction. ~ Given the fundamental role of the Diels-Alder reaction in organic synthesis [I], it is without question that the control of its absolute topicity constitutes a topic of pressing importance. It was, however, not until recently that substantial effort has been devoted to and hence significant progress achieved in this area [2].As an extension of a preliminary communication [3], we report here in detail the first successful surmounting of the threshold of 99 % face-stereodifferentiation in the Diels-Alder process. Its value is illustrated by application to the syntheses of enantiomerically pure building blocks and also of a natural product. The starting point for this work was the encouraging chiral induction observed with Lewis-acid-promoted ene-type cyclizations (1h2) [4] and [4+2] cycloadditions (3-4) [5] of 8-phenylmenthyl enoates

In the Quest for a Virtual Pseudo Receptor for Sandalwood‐Like Odorants. Part I

“…Finally, the peripheral position of the considered receptors limits the number of chiral or other aqueous/lipophilic barriers. 6 ) Depending on the geographical origin of the pine trees, the extracted a-pinenes may be of opposite absolute configuration [34]. 7 ) After our study and contrasting with our reported results (see Footnote 2), Buchbauer et al published that (À)-Madrol ¾ (1S,E)-10a was more powerful than its antipode [36].…”

“…Resulting from its increasing scarcity and cost, the search for synthetic substitutes has become, since several decades, the subject of various worldwide research activities [3], including those of Demole [4] and Schulte-Elte et al [5] at Firmenich SA. After an asymmetric synthesis of (À)-b-santalene [6], I was thus naturally interested in pursuing the initial efforts devoted to the understanding of the parameters and features imparting this elegant, precious-woody scent of great tenacity. The preliminary concept and results reported here were developed and performed from 1987 to 1993.…”

Diastereoselective 1,3‐Dipolar Cycloadditions of Chiral Derivatives of 2‐Oxoethanenitrile Oxide to Noncyclic Conjugated Symmetrical Alkenes

“…90 (2007) 2123 11 ) The reactive thermodynamically more stable conformation of these esters is assumed to have the carbonyloxy moiety syn-periplanar with respect to the CÀH bond of the hydroxylic center [35], as exhibited by the X-ray structure analysis of (4R,5R)-6b (Fig. 2, C(1)ÀH).…”

Two-fold tandem acyl-group shift/cyclization via gold catalysis

“…Installation of the allene moiety was performed by reaction of ylide 3 with ketene, generated in situ from acyl chloride and triethylamine. [22][23][24][25][26][27] Allenoyl derivative 4 could thus be obtained in 80% yield from α-bromoester 3. Final desilylation by means of potassium fluoride delivered 5 possessing the desired terminal triple bond in 63% yield.…”