Montanine

A stereospecific intramolecular imino-ene reaction was used by Weinreb and co-workers92 to provide the enantio-selective total syntheses of (-)-montanine, (-)-coccinine, and (-)-pancracine. Refluxing the imine resulting from the condensation of 151 and 152 in mesitylene produces the amine 153 (Scheme 33) as a single stereoisomer in 63% yield after removal of the silyl group from the alkyne. The high stereoselectivity is thought to arise from a concerted ene process. 

Kokas OJ, Banwell MG, Willis AC (2008) Chemoenzymatic Approaches to the Montanine Alkaloids A Total Synthesis of (+)-Nangustine. Tetrahedron 64 6444... 

The ketone 446 also served as a suitable starting material for the syntheses of ( )-montanine(415) and ( )-coccinine (413). Accordingly, the olefin 448 admixed with the A1 2 isomer was obtained from the tosylhydrazone 447 by base catalysed elimination. Bis-hydroxylation of the double bond of purified 448 achieved by the usual osmylation protocol... 

Scheme 62. Synthesis of ( )-coccinine, ( )-pancracine, ( )-montanine (continued next page).

Montanine, via Alder-ene reactions, 10, 593 Montmorillonite, in dichalcogenide additions, 10, 756 Morpholine, in trinuclear Ru and Os clusters, 6, 728 Morphology studies... 

The alkaloids related to crinine (359), which possess the 5,10b-ethanophen-anthridine nucleus, have been the subject of extensive synthetic investigations (190). Alkaloids of this family bearing oxygen functionality at C-ll occupy a special position since they are possible biosynthetic and chemical precursors of the subgroups that incorporate a 2-benzopyrano[3,4-c]indole ring such as those alkaloids related to pretazettine (395), tazettine (397), and macronine (401) and the methanomorphanthridines such as montanine (584). 

Although the alkaloids of the montanine type have not stimulated a high degree of synthetic interest, several approaches to montanine (584) have been... 

Montanine alkaloids are members of the alkaloids of the plant family of Amarylli-daceae. They represent a relatively small class of compounds with the typical methanomorphanthridine skeleton exhibiting some anxiolytic, anti-depressive and anti-convulsive activities [135, 136]. The total syntheses of the natural (-)-mon-tanine enantiomers are only mentioned, not described, here (for these (—(-enantiomers see, e.g. literature cited in [137]). 

The total chemo-enzymatic synthesis of three of the (-l-)-montanine alkaloids, namely (+)-brunsvigine [138], (-l-)-nangustine [139] and (-l-)-montabuphine [140], has been reported in the last 3 years. These non-natural enantiomers have been prepared from monochiral 3-halo-m-1,2-dihydrocatechols precursors. A total of over 17-20 reaction steps are needed for their syntheses (Fig. 22). 

Fig. 22 Chemo-enzymatic, enantio-selective total synthesis of three non-natural montanine type alkaloids, (+)-brunsvigine, (+)-nangustine and (+)-montabuphine (TD toluene dioxygenase)...

Amaryllidaceae Alkaloids.—Detail has been added to the fairly thoroughly delineated pathways to the Amaryllidaceae alkaloids.75 76 11-Hydroxyvittatine (94), previously shown to be a precursor for narciclasine (96),76 has been proposed as an intermediate in the biosynthesis of haemanthamine (92) and montanine (95) following the observed specific incorporation of vittatine (93) into the two... 

---