Neue Synthese von (−)‐(R)‐Cembren A, Synthese von (+)‐(R)‐Cembrenen und (+)‐(S)‐Cembren

Abstract

Novel Synthesis of (−)‐(R)‐Cembrene A, Synthesis of (+)‐(R)‐Cembrenene and (+)‐(S)‐Cembrene A novel synthesis of (−)‐(R)‐cembrene A ((−)‐3) was developed using the Sharpless epoxidation for the introduction of the chiral center. Furthermore, the synthesis of (+)‐(R)‐cembrenene ((+)‐4) showed that this cembranoid must have the (R)‐configuration and not, as previously reported, the (S)‐configuration. Selective hydrogenation of (+)‐4 afforded (+)‐(S)‐cenibrene ((+)‐5).

A Flexible Common Approach to α‐Substituted Serines and Alanines: Diastereoconvergent Syntheses of Sphingofungins E and F

“…Hydrogenolysis in the presence either of Pearlman's catalyst or of Raney nickel28 was first tested, but no conversion was observed. The mild reductant NaBH 3 CN29 in THF/HMPA (4:1) was also ineffective. Ni‐mediated reduction30 resulted in a complex mixture, presumably due to reductive opening of the adjacent epoxide.…”

Diterpene Biosynthesis from Geranylgeranyl Diphosphate Analogues with Changed Reactivities Expands Skeletal Diversity

“…[32] Unfortunately, this method is not always suitable for the determination of absolute configurations of macrocyclic ring compounds, because their conformational flexibility sometimes does not allow for an unambiguous interpretation of NOESY spectra. However, the absolute configurations of 1 and 2 could be tentatively assigned from their optical rotations [33] and (R)-4 ([α] D 25 = À 14.2, c 0.48, CHCl 3 ) [34] from plants. For 5 NOESY based assignments of diastereotopic hydrogens were possible (Figure S19), and isotopic labelling experiments with (E)-and (Z)-(4-13 C,4-2 H)IPP (Figure S83) pointed to the absolute configuration as shown in Scheme 1.…”

Diterpenbiosynthese aus Geranylgeranyldiphosphat‐Analoga mit veränderter Reaktivität erweitert die Diversität der Skelette

“…Leider ist diese Methode nicht immer zur Bestimmung absoluter Konfigurationen von makrocyclischen Ringverbindungen geeignet, da ihre konformative Flexibilität manchmal keine eindeutige Interpretation von NOESY‐Spektren zulässt. Die absoluten Konfigurationen von 1 und 2 konnten jedoch vorläufig aus ihren optischen Drehungen ( 1 : [α] D 25 =+11.4, c 0.07, Me 2 CO, 2 : [α] D 25 =+84.6, c 0.13, Me 2 CO) abgeleitet werden, die entgegengesetzte Vorzeichen zu den berichteten optischen Drehungen von ( R )‐ 3 ([α] D 25 =−46.3, c 1.20, CHCl 3 ) [33] und ( R )‐ 4 ([α] D 25 =−14.2, c 0.48, CHCl 3 ) [34] aus Pflanzen aufweisen. Für 5 waren NOESY‐basierte Zuordnungen von diastereotopen Wasserstoffatomen möglich (Abbildung S19), und Isotopenmarkierungsexperimente mit ( E )‐ und ( Z )‐(4‐ 13 C,4‐ 2 H)IPP (Abbildung S83) deuteten auf die in Schema 1 gezeigte absolute Konfiguration hin.…”