Pauson−Khand Reaction on the 1,2-Isopropylidenedioxyfuranoside Scaffold:  Expedient Access to a Chiral Cyclopenta[c]pyran Ring System

Cascade Reactions Involving Pauson—Khand and Related Processes

“…Examples include carbohydrate derivatives like 40 that give 41 with variable yields depending on the substitution pattern. 41 is transformed into cyclopenta[c]pyrane 42, which is the skeleton of iridoids [93]. In another example epichlorhydrin (43) is used to construct chiral enyne 44 which gives cyclopentenone 45 [94] (Scheme 14).…”

“…The formation of 93 is believed to be a consequence of the ionization of the propargylic C -N bond at the cobaltacycle step. The crowded metallacycle formed after the insertion of the olefin (93), would prompt the cleavage of the C -N bond, forming an ionic species (94) that would trap a hydride, possibly from a cobalt hydride giving 95, which then would follow the usual pathway towards the cyclopentenone (Scheme 27) [124]. …”

Acetylenosaccharides

“…The Pauson-Khand reaction [245], a Co 2 (CO) 8 -promoted transformation of enynes into cyclopentenones, has been applied to acetylenosaccharides [244,[246][247][248]. The cyclopentenones 211 and 212 were thus obtained from the enynes 210 and 208, respectively (Scheme 5.46).…”

“…The 5-exo-dig alkynol cycloisomerization is favored by cumulated triple bonds, by the presence of a 1-bromo, 1-chloro, or 1-(het)aryl substituent or of an additional ring in the starting material (as in 39), and by proximity of the reacting groups [22,199,250,251]. Activation of an alkynyl group resulting from coordination also promotes its reaction with nucleophiles; pent-4-yne-1,3-diol was thus transformed into 2-methylene-oxolan-3-ol by Lewisacid catalysis (Ag 2 CO 3 in refluxing benzene; 99 % yield [248]). …”

Expeditious synthesis of enantiopure symmetrical macroheterocycles by ring-closing metathesis of ether and tether-linked 1,2-O-isopropylidenefuranosides

“…Recently the 1,2-O-isopropylidenefuranose scaffold, readily available from the well-known 1,2:5,6-di-O-isopropylidene-a-D D-glucofuranose (2) (Scheme 2), has provided the template for a variety of reactions resulting in the synthesis of diverse types of enantiomerically pure skeletons. [26][27][28] The use of this carbohydrate scaffold will enable the future conjugation with other molecules via glycosylation at the anomeric centre. Another advantage of the occurrence of the O-isopropylidene protected furanoside rings in the macrocycles B is the future conversion of this ring system to a nucleoside.…”