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The additions of the enantiomerically pure organozinc reagents 17 and 33 to the THF-aldehyde 1 in the presence of the monodentate Lewis acid boron trifluoride-ether give the nonchelation-controlled addition products 7 and 36, respectively (stereoselectivity 95:5, 86: 14). These results provide a route to oligo(tetrahydr0furan)s with the relative stereochemistry trans-syn-cis. A stereodirecting effect of the chiral center in the organozinc reagent 17 is found, leading to simple diastereoselectivies in the reaction with achiral aldehydes and to a matched-mismatched case in the reaction with the chiral aldehyde 1.During our studies directed to the stereoselective synthesis of natural"] and non-natural[21 oligo(tetrahydr0fur-an)s (oligo-THFs) we used a chelation-controlled Grignard reaction as a synthetic route to THF dimers and trimers with the relative configuration tr~ns-anti-trans [~]. Cu(1)-catalyzed addition of the enantiomerically pure Grignard reagent 2 to the THF-aldehyde 1 gave the alcohol 3 with a stereoselectivity of 93:7. The acetonide functionality of 3 was stereoselectively transformed into the epoxide function of the alcohol 4. An intramolecular epoxide opening was used to close the new THF ring, thus providing the THF dimer 5 (relative configuration: trans-anti-trans). In order to realize a stereoselective synthesis of oligoTHFs with the relative configuration trans-syn-cis, it would be very eficient to use the same route as in Scheme 1, but a nonchelation-controlled addition step instead of the chelation-controlled one. This suggests the use of another organometallic reagent as the Grignard compound, i.e. we wanted to control the stereochemical outcome of the addition reaction by metal tuning. In this way the new organometallic reagent 6 should add to the THF-aldehyde 1 to give an acetonide alcohol 7. Transformation of the acetonide into the epoxide 8 and intramolecular ring closure would provide access to a THF dimer 9 with the relative configuration trans-syn-cis. Which metal is the right one, to achieve nonchelationcontrol in this particular situation? In this paper we report on synthetic studies to answer this question. The choice of the metal has to be based on the of stereoselective additions of organometallic reagents R-M to chiral a-alkoxy aldehydes of type 11, leading to the anti product 12 via a chelation-controlled pathway and to the syn product 10 via a nonchelation-controlled pathway. Introduced by CramF51, chelation control was a transition-state concept in its beginning. Nowadays, it is exper-
The additions of the enantiomerically pure organozinc reagents 17 and 33 to the THF-aldehyde 1 in the presence of the monodentate Lewis acid boron trifluoride-ether give the nonchelation-controlled addition products 7 and 36, respectively (stereoselectivity 95:5, 86: 14). These results provide a route to oligo(tetrahydr0furan)s with the relative stereochemistry trans-syn-cis. A stereodirecting effect of the chiral center in the organozinc reagent 17 is found, leading to simple diastereoselectivies in the reaction with achiral aldehydes and to a matched-mismatched case in the reaction with the chiral aldehyde 1.During our studies directed to the stereoselective synthesis of natural"] and non-natural[21 oligo(tetrahydr0fur-an)s (oligo-THFs) we used a chelation-controlled Grignard reaction as a synthetic route to THF dimers and trimers with the relative configuration tr~ns-anti-trans [~]. Cu(1)-catalyzed addition of the enantiomerically pure Grignard reagent 2 to the THF-aldehyde 1 gave the alcohol 3 with a stereoselectivity of 93:7. The acetonide functionality of 3 was stereoselectively transformed into the epoxide function of the alcohol 4. An intramolecular epoxide opening was used to close the new THF ring, thus providing the THF dimer 5 (relative configuration: trans-anti-trans). In order to realize a stereoselective synthesis of oligoTHFs with the relative configuration trans-syn-cis, it would be very eficient to use the same route as in Scheme 1, but a nonchelation-controlled addition step instead of the chelation-controlled one. This suggests the use of another organometallic reagent as the Grignard compound, i.e. we wanted to control the stereochemical outcome of the addition reaction by metal tuning. In this way the new organometallic reagent 6 should add to the THF-aldehyde 1 to give an acetonide alcohol 7. Transformation of the acetonide into the epoxide 8 and intramolecular ring closure would provide access to a THF dimer 9 with the relative configuration trans-syn-cis. Which metal is the right one, to achieve nonchelationcontrol in this particular situation? In this paper we report on synthetic studies to answer this question. The choice of the metal has to be based on the of stereoselective additions of organometallic reagents R-M to chiral a-alkoxy aldehydes of type 11, leading to the anti product 12 via a chelation-controlled pathway and to the syn product 10 via a nonchelation-controlled pathway. Introduced by CramF51, chelation control was a transition-state concept in its beginning. Nowadays, it is exper-
(?‐Tuberolacton (XI) und (iHasminlacton (XII) werden nach angegebenem Schema dargestellt (z.T.
Dedicated to Professor George Buchi on the occassion of his 60th birthday (ll.V.81) SummaryEasily accessible dihydropyrans 9, 10, 12 and 19 are precursors for the synthesis of 6-substituted 5,6-dihydro-2 (2H)-pyranones and 6-substituted tetrahydro-2-pyranones. Syntheses of massoia lactone (3), argentilactone (5), tuberolactone (4) and jasmine lactone (2) from acrolein (6), acrolein dimer (7) or glutaraldehyde (16) are described.Introduction. -Saturated and unsaturated aliphatic &lactones occur in several food flavors and essential oils [l] as the metabolites of higher molecular weight unsaturated fatty acids. Owing to their specific odor impression and low threshold concentration they play an important role as flavoring materials, and therefore practical syntheses of these d-lactones are greatly demanded.Whereas 5-decanolide (1) as well as its homologs are readily accessible from the Baeyer-Villiger reaction of the corresponding 2-substituted cyclopentanones, this approach is cumbersome for lactones containing C, C-double bonds (cf. syntheses of jasmine lactone (=(7Z)-7-decen-2-olide; 2) [2]). Despite the relatively simple structure of 6-substituted 5,6-dihydro-2 (2H)-pyranones, only a few syntheses to selected lactones are known [3]. We therefore looked for a general synthetic approach to ( f )-massoia lactone (= 2-decen-5-olide; 3), ( f )-tuberolactone (= (72)-2,7-decadien-Solide; 4), and (f )-argentilactone (= (62)-2,6-dodecadien-5-olide; 5). The latter lactone 5 was isolated recently from the rhizomes of Aristolochia argentina [4], and its synthesis has not yet been reported.
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