• Mediterranean‐type ecosystems contain 20% of all vascular plant diversity on Earth and have been identified as being particularly threatened by future increases in drought. Of particular concern is ...the Cape Floral Region of South Africa, a global biodiversity hotspot, yet there are limited experimental data to validate predicted impacts on the flora. In a field rainout experiment, we tested whether rooting depth and degree of isohydry or anisohydry could aid in the functional classification of drought responses across diverse growth forms. • We imposed a 6‐month summer drought, for 2 yr, in a mountain fynbos shrubland. We monitored a suite of parameters, from physiological traits to morphological outcomes, in seven species comprising the three dominant growth forms (deep‐rooted proteoid shrubs, shallow‐rooted ericoid shrubs and graminoid restioids). • There was considerable variation in drought response both between and within the growth forms. The shallow‐rooted, anisohydric ericoid shrubs all suffered considerable reductions in growth and flowering and increased mortality. By contrast, the shallow‐rooted, isohydric restioids and deep‐rooted, isohydric proteoid shrubs were largely unaffected by the drought. • Rooting depth and degree of iso/anisohydry allow a first‐order functional classification of drought response pathways in this flora. Consideration of additional traits would further refine this approach.
A novel approach to medium-sized cyclic ethers was devised using a Stevens 1,2-shift of a sulfonium ylide derived from a readily accessible six-membered mixed-monothioacetal precursor. The concise ...and efficient transformation offers a surprising degree of chirality transfer with observed retention of stereochemical configuration on the anomeric migrating carbon and has been applied as the key step in an enantioselective formal synthesis of (+)-laurencin.
1,2-Cycloheptadiene is a strained, transient species that has been underutilized as a synthetic building block. Seven-membered cyclic allenes are mostly known for their propensity to undergo rapid ...dimerization, and relatively little has been reported regarding their cycloaddition reactivity with 1,3-dienes or 1,3-dipoles. This work describes the trapping of 1-acetoxy-1,2-cycloheptadiene and its unsubstituted counterpart, generated via desilylative elimination, with a range of 1,3-dipolar trapping partners, affording complex polycyclic products with high regio- and diastereoselectivity.
Intramolecular 4 + 2 cycloaddition reactions of substituted 1,2-cyclohexadienes with pendent furans enables the synthesis of complex tetracyclic scaffolds in a single step under mild conditions. All ...Diels–Alder cycloadducts were obtained as single diastereomers, assigned as the endo isomer. Substrates were easily assembled via Stork–Danheiser alkylation of 3-ethoxy-2-bromocyclohex-2-enone to accommodate a range of tethers and furan traps. Cleavage of enol acetate moieties resulted in room-temperature Diels–Alder cycloreversion to tethered furyl cyclohexenones.
Oxygenated-1,2-cyclohexadienes and their unsubstituted counterpart can be generated under mild conditions by fluoride-induced desilylation and undergo intermolecular 2 + 2-cycloaddition reactions ...with a variety of alkene traps to afford bicyclo4.2.0octenes. Both styrenes and electron-deficient olefins react in good conversion and with complete regioselectivity in favor of cyclobutane formation at the unsubstituted C2/C3 carbons of the C1-substituted cyclic allenes. Diastereoselectivities are modest (1.1–5.7:1) with a preference for the exo-isomer.
3‐Acyl‐4‐hydroxypyan‐2‐ones with pendent furans, readily available from dehydroacetic acid, are substrates for intramolecular 4+4‐photocycloaddition. In the presence of stoichiometric lithium base, ...these substrates undergo conversion to lactone‐bridged cyclooctenones in moderate to good yields upon irradiation in THF at 0 °C. In contrast to other crossed 4+4‐photocycloadditions of pyran‐2‐ones, these examples proceed with complete diastereoselectivity for the endo cycloadducts.
Readily accessible 3‐acyl‐4‐hydroxypyran‐2‐ones with pendent furans undergo 4+4‐photocycloaddition reactions in the presence of lithium ion to afford lactone‐bridged cyclooctenones with complete selectivity for the endo cycloadducts.
A strained ring for stereoselective synthesis Jankovic, Christian L; West, F G
Science (American Association for the Advancement of Science),
2023-Jan-20, 2023-01-20, 20230120, Letnik:
379, Številka:
6629
Journal Article
Recenzirano
The spring-loaded tension of a cyclic allene expedites synthesis of a natural product.
A short, stereoselective approach to the fused tricyclic carbon skeleton found in the tigliane and daphnane classes of diterpene natural products is described. Convergent coupling of the A- and ...C-rings, followed by diastereoselective cerium enolate addition and formation of a double acetal set the stage for generation of an oxonium ylide via a transient metallocarbene. An efficient Stevens 1,2-shift furnished the 7-membered B-ring, possessing the bridgehead oxygenation pattern found in the natural systems.
Oxidation‐initiated Nazarov reactions of 1,4‐pentadien‐3‐yl ethers take place in the presence of DDQ. Termination by regioselective elimination preserves both stereocenters created during ...electrocyclization, providing cyclopentanone products bearing an exocyclic methylene unit. Use of catalytic DDQ with MnO2 as terminal oxidant is also described.
Dehydrogenative Nazarov reaction: 1,4‐Pentadien‐3‐yl ethers undergo oxidative conversion to the corresponding 3‐alkoxypentadienyl cations followed by clean Nazarov cyclization and regioselective exocyclic elimination to furnish alkylidene cyclopentenol ethers with the net loss of H2. This method provides mild, Lewis acid‐free access to Nazarov products in which the stereogenic centers formed in the electrocyclization are preserved.
Keto-substituted 1,2-cyclohexadienes were generated by base-mediated (KO
t
-Bu) elimination, and found to dimerize
via
an unprecedented formal hetero-Diels-Alder process, followed by hydration. These ...highly reactive cyclic allene intermediates were also trapped in Diels-Alder reactions by furan, 2,5-dimethylfuran, or diphenylisobenzofuran to afford cycloadducts with high regio- and diastereoselectivity, and could also be intercepted in a hetero-Diels-Alder process with enamine dienophiles.
Endo
/
exo
stereochemistry was unambiguously determined
via
X-ray crystallography in the case of nitrile-substituted 1,2-cyclohexadiene. DFT calculations indicate that the novel hetero-Diels-Alder processes observed with these allenes occur
via
a concerted asynchronous cycloaddition mechanism.
Keto-substituted 1,2-cyclohexadienes undergo a novel dimerization process and can be trapped with various furan-based 1,3-dienes or enamine dienophiles.
