A Metathetic Approach to [5/5/6] Aza-Tricyclic Core of Dendrobine, Kopsanone, and Lycopalhine A Type of Alkaloids

Abstract

A concise synthetic approach to [5/5/6] tricyclic pyrrolidine core of dendrobine is reported. This methodology relies on the construction of β-hydroxylactams by NaBH4-I2 reduction followed by reaction of allylsilane with the aid of Lewis acid to generate alkenyl lactams in good yields. Further, ring-opening metathesis (ROM) followed by ring-closing metathesis (RCM) were used to assemble the [5/5/6] aza-tricyclic skeleton of dendrobine. This short synthetic route has been expanded to assemble tricyclic [5/5/8] system with pentenylboronic acid.

Application of New Efficient Hoveyda–Grubbs Catalysts Comprising an N→Ru Coordinate Bond in a Six-Membered Ring for the Synthesis of Natural Product-Like Cyclopenta[b]furo[2,3-c]pyrroles

“…However, it was found that the pure trans -isomers 6aA , 6eA and 6pA are able to form tricyclic systems of cyclopenta[ b ]furo[2,3- c ]pyrrole 7 in the RRM reaction (the last column of Table 2 and Table 4 ). Moreover, in contrast to the communications [ 9 , 10 , 11 , 17 , 19 , 21 , 22 ], in our case, the ring-rearrangement metathesis does not require the supporting application of ethylene that facilitates practical synthesis.…”

“…It worth mentioning here that by revising the ethylene-supported transformations of the similar substrates [ 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 ], the question of the sequence of metathesis stages arose. In theory, two paths exist for the rearrangement of 6 to 7 in an ethylene atmosphere: ROM/RCM, or vice versa, RCM/ROM successions [ 43 ].…”

“…2), with a shorter bond length between nitrogen and ruthenium (the N→Ru bond is 2.24 Å [27]), is stable at least up to 140 • C [27], and therefore turned out to be effective for the preparation of cyclopenta[b]furo [2,3-c]pyrrole 7p at temperatures above 100 • C in microwave-assisted experiments (entries 17-26, Table 4). It should be noted that the amount of catalysts 1 and 2 required for the complete conversion of the initial esters 6eA, 6pA did not exceed 0.5 mol % in both cases, while the reaction time was much shorter than in the papers cited above [9][10][11][12][13][14][15][16][17][18][19][20][21][22][23].…”

“…The discovery of metathesis reactions has allowed the noticeable expansion of possibilities for the transformation of different unsaturated substrates, including those aimed at obtaining complex naturally occurring compounds and pharmacologically meaningful molecules, which is reflected in a number of recent reviews [ 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 ]. In particular, the ring-rearrangement metathesis (RRM) of bi- and polycyclic alkenes has made it possible to obtain systems that are practically inaccessible by means of other synthetic pathways [ 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 ]. Most often, for the rearrangement of bridged alkenes, the commercially available Grubbs (G) and Hoveyda–Grubbs (HG) ruthenium catalysts are used, which permits the reliable production of scaffolds possessing a wide range of potential biological activity due to the characteristics of their carbon skeletons [ 19 , 20 , 21 , 22 , 23 ].…”

Application of Sequential Ring‐Opening, and Ring‐Closing Metathesis or Ring‐Rearrangement Metathesis to Design Oxacycles and Azacycles

“…Allylation [11a] of 7 with allyltrimethyl silane in the presence of Lewis‐acid such as BF 3 .OEt 2 at 0 °C furnished the allyl derivative 8 . The stereochemistry of the newly introduced allyl group in 8 is established based on our earlier reported results [11b] . Unfortunately, the treatment of the allyl derivative 8 with G‐II catalyst did not deliver the ring‐rearranged product 10 , however, ring‐opening metathesis product 9 was observed in the presence of Hoyeda ‐Grubbs’ first‐generation (GH‐I) catalyst in 72 % yield.…”