III. C-Nitro Carbohydrates

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A. Group Replacement

Since in the reaction of a C-nitro compound the stability of the developing radical (R·) affects the ease of cleavage of the carbon–nitrogen bond (Scheme 2), group replacement occurs more easily for tertiary nitro compounds⁹^–14 than for secondary¹⁵^–19 and, especially, primary ones.^20–22 Reaction of Bu₃SnH with a compound containing a tertiary nitro group is given in eq 1.⁹ If the nitro group in the substrate is secondary, reaction usually follows the same pathway and replaces this group with a hydrogen atom,^15–18 but sometimes breaking a C–O bond leading to a nitroso compound (which isomerizes to an oxime) offers significant competition (Schemes 2 and 5).¹⁹ When a nitro group is primary, the elimination phase of the reaction is more likely to produce only the nitroso compound (eq 2),^20,21 even though replacement of a primary nitro group with a hydrogen atom has been observed (eq 3).²²

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B. Addition Reactions

A nitro group in a reactant molecule can be involved in radical addition in several ways. First, denitration can produce a carbon-centered radical that undergoes typical addition to an electron-deficient multiple bond (eq 4).³ In a different role, nitro groups activate multiple bonds toward addition by nucleophilic radicals and affect the regioselectivity of such reactions (eq 5).²³ Deprotonation of a carbon atom bearing a nitro group creates an unsaturated system to which a carbon-centered radical can add to form a new, C–C bond (Scheme 6²⁴).^24–26 Finally, the electron-withdrawing character of a nitro group can contribute to turning a normally nucleophilic radical into one that is electrophilic; thus, the philicity of the radical 1, which has both nitro and ethoxycarbonyl groups attached to the radical center, is reflected in its ability to add to the electron-rich double bond in the glycal 2 (Scheme 7).²⁷

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C. Cyclization Reactions

A cyclization reaction that begins with a C‑nitro carbohydrate is shown in eq 6.²⁸ The high yield of this reaction, which involves a radical centered on a tertiary carbon atom adding to a multiple bond, illustrates that radical addition can be relatively insensitive to steric congestion at the radical center.^28,29

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D. Elimination Reactions

A deoxynitro sugar can undergo an elimination reaction, if a radical center develops on a carbon atom adjacent to that bearing the nitro group.^30–32 In the reaction shown in Scheme 8,³⁰ such a radical forms and then eliminates nitrogen dioxide to give an unsaturated compound.