Stereocomplementary Routes to Hydroxylated Nitrogen Heterocycles: Total Syntheses of Casuarine, Australine, and 7‐epi‐Australine. Issue 32 (4th July 2013)

• Record Type: Journal Article
• Authors: Parmeggiani, Camilla; Cardona, Francesca; Giusti, Leonardo; Reissig, Hans‐Ulrich; Goti, Andrea
• Is Part Of: Chemistry. Volume 19:Issue 32(2013)
• Page Start: 10595
• Page End: 10604
• Publication Date: 2013-07-04
• Subjects: Chemistry -- Periodicals; 540
• DOI: 10.1002/chem.201301320
• Languages: English
• ISSNs: 0947-6539

Abstract

<abstract abstract-type="main" xml:lang="en"> <title>Abstract</title> <p>Addition of lithiated 1‐benzyloxyallene to a <sc>D</sc>‐arabinose‐derived cyclic nitrone occurred with perfect diastereoselectivity furnishing a bicyclic 1, 2‐oxazine derivative, which is an excellent precursor for pyrrolizidine alkaloids hydroxylated at C‐7 with optional configuration at this stereogenic center. Depending on the stage of the NO bond cleavage and ring re‐closure, 7‐hydroxypyrrolizidines with 7<italic>R</italic> or 7<italic>S</italic> configuration were obtained, as a result of completely selective addition reactions occurring complementarily at the bottom or top face of the endocyclic CC double bond in six‐ and five‐membered B rings, respectively. Applicability of these stereodivergent routes to obtain polyhydroxy pyrrolizidine alkaloids is demonstrated by the efficient syntheses of casuarine and australine as examples of the two classes of diversely configured 7‐hydroxypyrrolizidine alkaloids. An alternative synthesis of australine and two strategies for the preparation of 7‐<italic>epi</italic>‐australine are also reported, which demonstrate that the stereoselectivity of hydride reduction of an exocyclic CO double bond is independent of the ring size, occurring preferentially from the top face either in a six‐ or five‐membered ring.</p> </abstract>