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As shown by X-ray crystallography, crystals of 3β-acetoxy-16,17-seco-17,20-dioxopregn-5-ene-16-nitrile are dimorphic. The regioselectivity of the Norrish–Yang type II photocyclization under visible light of this steroidal 1,2-diketone, which bears primary, secondary, and tertiary nonequivalent abstractable γ-hydrogens, dramatically increases in the crystalline state of both polymorphs. X-ray crystallography and molecular mechanics calculations reveal crystal structure–solid state photochemistry relationships., Financial support by the Investigation Programs of the Ministerio de Economía y Competitividad (CTQ2010-18244) is acknowledged. D.A.-D. thanks the Ministerio de Economía y Competitividad for a fellowship. The authors acknowledge support of the publication fee by the CSIC Open Access Publication Support Initiative through its Unit of Information Resources for Research (URICI).

A simple epimerization of C-(α-D-Gly p )methanol into C-(β-L-Gly p )methanol compounds is described. The radical sequence involved homolytic cleavage of the C5–H bond by 1,5-hydrogen atom transfer promoted by the 1’-O-yl radical and subsequent hydride addition with inversion of configuration. This methodology allows the preparation of rare C-(β-L-Ido p )-, C-(β-L-Altp)-, C-(β-L-Gulp)-, C-(β-L-Allp)methanol glycosides starting from carbohydrates of the D-series. It can also be applied to transform L- into D-configured C-glycosyl compounds, as illustrated by the formation of C-(β-D-6dGulp)- and C-(β-D-6dAltp)methanol from readily accessible L-rhamno and L-fuco analogs, respectively. In further development of this procedure compounds with C-(β-L-Araf)- and C-(β-L-5dRibf)methanol structures have also been synthesized. The alkoxyl radicals were generated by reaction of the corresponding N-alkoxyphthalimides with nBu3SnH(D) and in comparative terms, by visible light-photocatalysis using the Hantzsch ester/fac-Ir(ppy)3 procedure. The influence of the sugar ring conformation and the electronegativity of the substituents on the stereochemical outcome will be addressed., The authors gratefully acknowledge the financial support from the Investigation Programs of the Ministerio de Economia y Competitividad (CTQ2010-18244), Fundación CajaCanarias (2015-BIO08), the European Regional Development Funds (ERDF) and the Gobierno de Canarias (ProID2017010017). ASM is grateful to the CSIC JAE Predoc Program (Spanish Research Council). The authors acknowledge support of the publication fee by the CSIC Open Access Publication Support Initiative through its Unit of Information Resources for Research (URICI).

A simple method to modify the primary face of cyclodextrins (CDs) is described. The 6I‐O‐yl radical of α‐, β‐, and γ‐CDs regioselectively abstracts the H5II, located in the adjacent D‐glucose unit, by an intramolecular 1,8‐hydrogen‐atom‐transfer reaction through a geometrically restricted nine‐membered transition state to give a stable 1,3,5‐trioxocane ring. The reaction has been extended to the 1,4‐diols of α‐ and β‐CD to give the corresponding bis(trioxocane)s. The C2‐symmetric bis(trioxocane) corresponding to the α‐CD is a stable crystalline solid whose structure was confirmed by X‐ray diffraction analysis. The calculated geometric parameters confirm that the primary face is severely distorted toward a narrower elliptical shape for this rim., Financial support by the Investigation Programs of the Ministerio de Economía y Competitividad (CTQ2010‐18244) and the COST Action CM1201 “Biomimetic Radical Chemistry” is acknowledged. D.A.‐D. thanks the Ministerio de Economía y Competitividad for a fellowship.

The excitation of the innermost carbonyl of nono‐2,3‐diulose derivatives by irradiation with visible‐light initiates a sequential Norrish type II photoelimination and aldol cyclization process that finally gives polyfunctionalized cyclopentitols. The rearrangement has been confirmed by the isolation of stable acyclic photoenol intermediates that can be independently cyclized by a thermal 5‐(enolexo)‐exo‐trig uncatalyzed aldol reaction with high diastereoselectivity. In this last step, the large deuterium kinetic isotope effect found for the 1,5‐hydrogen atom transfer seems to indicate that the aldol reaction runs through a concerted pericyclic mechanism. Owing to the ready availability of pyranose sugars of various configurations, this protocol has been used to study the influence of pyranose ring‐substituents on the diastereoselectivity of the aldol cyclization reaction. In contrast with other pyranose ring contraction methodologies no transition‐metal reagents are needed and the sequential rearrangement occurs simply by using visible light and moderate heating (0 to 60 °C)., This work was supported by the Investigation Programs (CTQ2010‐18244) of the Ministerio de Economía y Competitividad and (SolSub20081000192) of the Gobierno de Canarias. D.A.‐D. thanks the Ministerio de Economía y Competitividad for a fellowship. C.R.‐F. thanks the Programa I3P‐CSIC for a postdoctoral contract.

The nitrogen-centred radical generated by reaction of an N-phosphoramidate or N-cyanamide, attached to a tri- or tetramethylene tether extended from the C-1 of a carbohydrate, with (diacetoxyiodo)benzene (DIB) and iodine can undergo a regio- and stereoselective intramolecular hydrogen atom transfer (HAT) reaction to furnish four different oxa-azaspirobicyclic systems: 1-oxa-6-azaspiro[4.4]nonane, 1-oxa-6-azaspiro[4.5]decane, 6-oxa-1-azaspiro[4.5]decane and 1-oxa-7-azaspiro[5.5]undecane. A tandem 1,5- or 1,6-HAT-oxidation-nucleophilic cyclisation mechanism is proposed., This work was supported by the research programmes BQU2000-0650 and BQU2001-1665 of the Dirección General de Investigación Científica y Técnica, Spain. I.P.-M. thanks the I3P-CSIC programme for a fellowship.

The preparation of 2-amino- C -glycosides of the hexahydro-2 H -furo[3,2- b ]pyrrole and octahydropyrano[3,2- b ]pyrrole systems is described. A tandem 1,5-hydrogen atom transfer–radical oxidation–nucleophilic cyclisation mechanism is proposed for the intramolecular hydrogen atom transfer reaction promoted by carbamoyl and phosphoramidyl radicals employing hipervalent iodine oxidants.

The primary alkoxyl radical generated by reaction of 1-(2-hydroxyethyl)-glycosides with (diacetoxyiodo)benzene (DIB) and iodine can undergo regioselective intramolecular hydrogen abstraction (IHA) reactions to furnish four different dioxabicyclic systems derived from carbohydrates. The results strongly suggest that the regiocontrol and feasibility of the cyclisation are dependant not only on geometric and stereoelectronic factors, but also on polar and thermochemical factors. The correct selection of the substituents at the precursors can favour the 1,6-IHA against the 1,5-IHA pathway.

A simple synthesis of all eight C-22, C-23, and C-25 diastereoisomers of the cephalostatin north 1 side chain has been accomplished from (25R)-5α-spirostan-3β-ol (tigogenin). The synthesis involves selective hydroxylations at C-23 and C-25 and reductive opening of the 1,6-dioxaspiro[4.5]decane spirostan system to give a conveniently protected 5α-furostan-3β,23,25,26-tetrol. The construction of the required 1,6-dioxaspiro[4.4]nonane system entailed an intramolecular hydrogen abstraction reaction promoted by the C-25 alkoxyl radical as the key step. Acid-catalyzed isomerization of the spiroketal unit suggested that 22R isomers are the thermodynamic products while the 22S isomers are the result of kinetic control. The acid-catalyzed equilibrium between 1,6-dioxaspiro[4.4]nonane and 1,6-dioxaspiro[4.5]decane systems was also studied. In the 1,6-dioxaspiro[4.4]nonane units, the observed 3J23,24 coupling constants suggest that the five-membered puckered ring-F undergoes substantial conformational changes on going from 22S to 22R isomers.

A series of four α-diketones placed as 1α-pyruvoyl tethers on D-glucopyranose and D-glucopyranosiduronic acid skeletons was prepared in order to determine the influence of captodative and stereoelectronic effects on the regioselectivity of the hydrogen atom transfer (HAT) in Norrish typeII photochemical processes. We observed that the 1,5-HAT regioselectivity can be switched between the two potentially abstractable syn-1,3-diaxial hydrogens at H6 and H8. Highly unusual photoproducts from Norrish typeII photoelimination and Norrish-Yang photocyclization initiated by the excited internal carbonyl group were obtained, in some cases in excellent synthetic yield. The 1,5-HAT transition state in the Norrish typeII photoelimination was investigated by photochemical experiments in the crystalline state.

The excitation of the innermost carbonyl of nono-2,3-diulose derivatives by irradiation with visible-light initiates a sequential Norrish typeII photoelimination and aldol cyclization process that finally gives polyfunctionalized cyclopentitols. The rearrangement has been confirmed by the isolation of stable acyclic photoenol intermediates that can be independently cyclized by a thermal 5-(enolexo)-exo-trig uncatalyzed aldol reaction with high diastereoselectivity. In this last step, the large deuterium kinetic isotope effect found for the 1,5-hydrogen atom transfer seems to indicate that the aldol reaction runs through a concerted pericyclic mechanism. Owing to the ready availability of pyranose sugars of various configurations, this protocol has been used to study the influence of pyranose ring-substituents on the diastereoselectivity of the aldol cyclization reaction. In contrast with other pyranose ring contraction methodologies no transition-metal reagents are needed and the sequential rearrangement occurs simply by using visible light and moderate heating (0 to 60 degrees C).