The century old [2+2] photocycloaddition reaction appeared as the most synthetically useful reaction amongst all photochemical reactions. It provides not only cyclobutane ring systems, but at the same time it provides access to medium rings through facile ring expansion/fragmentation of the strained cyclobutane ring. The high regio‐and stereoselectivity observed during cycloaddition has made it an attractive tool for synthesis of multicyclic structurally complex natural products. The vast majority of [2+2] photocycloaddition reactions involves enone–alkene cycloaddition which is conveniently achieved through direct excitation or sensitization by UV irradiation. Substantial progress has also been made in [2+2] cycloaddition between two unactivated alkenes using transition metal salts especially copper(I) salts. Sometimes Cu(I)‐catalyzed cycloaddition is advantageous over enone–alkene photocycloaddition. Search for an alternative source to environmentally harmful high energy UV light has led to the discovery of transition metal salts which can be used to excite alkenes by visible light. This causes alkenes to undergo [2+2] cycloaddition and has been used for synthesis of cyclobutane derivatives. This review aims to give an overview about different aspects of [2+2] photocycloaddition reaction with an emphasis in the stereoselective synthesis of selected complex natural products and related molecules where cycloaddition has been used as the key step.

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Polyols or sugar alcohols, a group of reduced calorie sweeteners, are the natural and nutritive sweeteners. These are neither sugars nor alcohols; rather they are a group of low-digestible carbohydrates which can be used instead of sucrose. They occur naturally in foods and come from plant products such as fruit and berries. They are used in food as sweeteners and bulking agents. Polyols have slightly reduced sweetness and caloric values compared to sucrose. Polyols available as either in solid crystalline form or syrups are emerging as a sugar replacer as well as a sugar substitute. Low sugar or low calorie is a top ranked market trend for bakery sector. Polyols offer the baker a versatile range of ingredients to boost the available portfolio of products. They would provide the functional benefits to bakery goods when the sugars used are replaced with polyols. This review focuses on some recent studies carried out on sucrose replacement with polyols in baked products.

Ring-closing metathesis has been applied to a series of glucose derivatives to produce cyclopentene derivatives 5a and 5b, cyclohexene derivatives 8 and 9, cycloheptene 12, and cyclooctene 14. Spirocyclic dihydrofurans 19, 26a, and 26b, along with dihydropyran 22, were also produced. A range of fused oxepine derivatives 29a-c and one oxo-cyclononene 31 were also prepared. Cyclopentene 5b was subjected to a sequence of hydrogenation, NBS bromination, and treatment with powdered zinc to furnish the ring-expanded product 35. No such ring expansion occurred when the cyclohexaannulated compound 8 was treated with NBS followed by powdered zinc, leading to aldehyde 39. The spiro dihydrofuran derivative 19 was converted to the aldehyde 42 via the same reaction sequence used to fragment cyclopentene derivative 5b.

[reaction: see text] A simple approach for asymmetric induction in Cu(I)-catalyzed [2 + 2] photocycloaddition, where asymmetric catalysts or chiral auxiliaries were inefficient, has been developed using the concept of chirality transfer from the readily available 2, 3-di-O-cyclohexylidine-(R)-(+)-glyceraldehyde. An anion-induced cleavage of the tetrahydrofuran ring of the resulting oxa-bicyclo[3.2.0]heptanes led to a convenient access to the synthetically useful cis-1,2-disubstituted cyclobutanes in enantiomerically pure form.