The stereoselective total synthesis of the sesquiterpene herbertenediol (3) and of its naturally occurring
dimers, mastigophorenes A [(P)-1] and B [(M)-1], is described. Following the “lactone concept”, the
configuration at the biaryl axis was atropo-divergently induced to be P or, optionally, M, by stereocontrolled
reductive ring cleavage (diastereomeric ratio up to 97:3) of the configurationally unstable joint biaryl lactone
precursor 17 using the oxazaborolidine−borane system, through dynamic kinetic resolution. Mechanistic
considerations of the lactone coupling suggested interference by a methoxy group next to the halogen substituent
and led to an improvement of the coupling yield from 39 to 87% (to give the lactone 37). As a new, likewise
highly efficient variant of the lactone method, we report for the first time thenow nondynamickinetic
resolution of a structurally related, but centrochiral “aliphatic−aromatic” lactone, (rac)-10. Its highly efficient
(k
rel > 300) enantiomer-differentiating Corey−Bakshi−Shibata reduction delivers the centrochiral building
block (R,R)-10 in good chemical yield and with excellent stereochemical purity (enantiomeric excess > 99.9%;
enrichment of the starting matrial). The new synthesis of natural herbertenediol (3) confirms its absolute
stereostructure as well as that of its dimers, (P)-1 and (M)-1.
Aim The cosmopolitan genus Herbertus is notorious for having a difficult taxonomy and for the fact that there is limited knowledge of species ranges and relationships. Topologies generated from variable molecular markers are used to discuss biogeographical patterns in Herbertus and to compare them with the geological history of continents and outcomes reported for other land plants.Location Africa, Asia, Azores, Europe, southern South America, northern South America, North America, New Zealand.Methods Phylogenetic analyses of nuclear ribosomal internal transcribed spacer and chloroplast (cp) trnL-trnF sequences of 66 accessions of Herbertus and the outgroup species Triandrophyllum subtrifidum and Mastigophora diclados were used to investigate biogeographical patterns in Herbertus. Areas of putative endemism were defined based on the distribution of species included in the analyses. Maximum parsimony analyses were undertaken to reconstruct ancestral areas and intraspecies migration routes.Results The analyses reveal species-level cladograms with a correlation between genetic variation and the geographical distribution of the related accessions. The southern South American Herbertus runcinatus is sister to the remainder of the genus, which is split into two main clades. One contains the Neotropical-African Herbertus juniperoideus and the New Zealand/Tasmanian Herbertus oldfieldianus. An African accession of H. juniperoideus is nested within Neotropical accessions. The second main clade includes species that inhabit Asia, the Holarctic, Africa, and northern South America. Maximum parsimony analyses indicate that this clade arose in Asia. Herbertus sendtneri originated in Asia and subsequently colonized the Holarctic and northern South America. An Asian origin and colonization into Africa is indicated for H. dicranus.
Main conclusionsThe current distribution of Herbertus cannot be explained by Gondwanan vicariance. A more feasible explanation of the range is a combination of short-distance dispersal, rare long-distance dispersal events (especially into regions that faced floral displacements as a result of climatic changes) extinction, recolonization, and diversification. The African Herbertus flora is a mixture of Asian and Neotropical elements. Southern South America harbours an isolated species. The molecular data indicate partial decoupling of molecular and morphological variation in Herbertus. Biogeographical patterns in Herbertus are not dissimilar to those of other groups of bryophytes, but elucidation of the geographical ranges requires a molecular approach. Some patterns could be the Journal of Biogeography (J. Biogeogr.) (2007) 34, 688-698 688
Complementary information on these compounds has been obtained from lH, 13C, and l g F n.m.r., including double and triple resonance experiments. The coupling constant, ,J(PNP), covers the range -23.9 to 731.9 Hz, and its magnitude and sign can be related to the proportion of molecules in which the local symmetry of the P-N-P skeleton is C,, rather than C,. Diphosphinoamine conformers of the latter type have been clearly identified for the first time both in solution (for R = Et or Pr', X = Ph, and R = But or CH,But, X = CI) and also in the solid phase.X-Ray analysis of Ph,PN(Pr') PPh,, based on 1 269 diffractometric intensities refined to R = 0.035, shows that the symmetry of the P-N-P skeleton is close to C, in the solid, with P-N = 1.706(4) and 1.71 l ( 4 ) A, and PNP = 122.8(3)".
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