Reduction of the steroidal sapogenin spiroketal system

Abstract

Reduction of tigogenin (la) and deoxytigogenin (lb) with lithium aluminum deuteride-aluminum chloride was shown to involve incorporation of hydride at the 22 position by an intermolecular mechanism. A possible alternative involving an intramolecular hydride shift from the 26 position was excluded. Position of the deuterium label was conclusively shown by pmr measurements and by chemical methods. An attempt to employ mass spectral evidence for assigning the deuterium position led to the observation of a rare seven-center transition state in an electron impact fragmentation sequence.

Redox Refunctionalization of Steroid Spiroketals. Structure Correction of Ritterazine M

“…The reaction takes the same stereochemical course regardless of configuration or substituents at C-12/C-14 ( 5a , 5b , 8 ) or C-25/C-26 ( 11a , 11b (Scheme ). It is noteworthy that this mild, selective method reduces 5/5 spiroketals (1,6-dioxaspiro[4.4]nonanes), in contrast to LAH/AlCl 3 8b. Only 22αD compounds were obtained with Et 3 SiD, confirming that the internal redox process was inoperative .…”

“…Woodward's internal redox mechanism for C-25 epimerization via C-26 to C-22 hydride transfer as adopted by Pettit for the LAH-mediated reduction of steroidal spiroketals (Scheme ) has so cemented synthetic thinking that the possibility of direct hydride addition to a C-22 oxonium ion is seldom considered.…”

Strategies to Control Alkoxy Radical-Initiated Relay Cyclizations for the Synthesis of Oxygenated Tetrahydrofuran Motifs

“…Alkoxy radical 44 (Figure ) may first undergo a 1,5-hydrogen atom transfer, followed by a 6- exo cyclization, to form tetrahydropyran derivative 46 (Path A). Alternatively, the oxygen of the ether should be sufficient to bias the system to a preferential 1,6-HAT ,,,, forming radical intermediate 47 , which would then cyclize to form tetrahydrofuran 48 (Path B). Using the previously optimized relay cyclization conditions, precursor 42 cyclized to tetrahydrofuran 48 in 66% isolated yield as an 80:20 ratio of isomers (Scheme )…”

Zur Prins‐Reaktion von Lumihecogeninacetat

“…Nach Trocknen der Chloroform-Extrakte, Eindampfen und SC (Petrolether/Essigester 3: 1) wurden 1.08 g (31%) 7 a und 0.63 g (18%) 10a erhalten. Bortrifluorid (1/1) (20); 935 (86); 916 (7); 802 (10); 784 (17); 471 (25); 456 (68); 453 (100); 139 (22); 126 (89); 115 (11). 3~-Acetoxy-Sa-spirost-14-en-12a-o14s) 358 (73, d ) ; 126 (100, I ) .…”