= Solution
The <lactol> is a cyclic <hemiacetal> and can open to \b[$\mathrm{HOCH_2CH_2{-}CH(OBn){-}CHO}$], retaining its pre-existing alkoxy-bearing stereocentre. Its two <anomers> lose their anomeric configuration on opening to the planar <aldehyde>, so both can feed the same addition pathway. A first equivalent of decylmagnesium <bromide> can be consumed by the free OH, producing decane and a <magnesium> alkoxide; the excess <reagent> supplies the <carbon> <nucleophile> for addition.
<magnesium> can coordinate both the <aldehyde> <oxygen> and the adjacent benzyloxy <oxygen>. This produces a five-membered chelate, with ring sequence Mg–O(<carbonyl>)–C(<aldehyde>)–C(alpha)–O(benzyloxy), that restricts rotation about the <aldehyde>–alpha-<carbon> bond. Decyl attack on the less crowded face, away from the alpha-<carbon>'s hydroxyethyl chain in the chelated arrangement, forms the new C–C bond and a <magnesium> alkoxide at the former <aldehyde> <carbon>. Acidic workup protonates both alkoxides to give G. The OBn bond is retained; it is not displaced by the <Grignard reagent>.
\b[The <product (chemistry)> configuration is the one with OBn and the newly formed OH both projecting forward in the specified zigzag chain], while the opposite configuration at the new <alcohol> <carbon> is disfavoured. This is the <Cram chelation model> for an alpha-alkoxy <aldehyde>, rather than <stereochemical> memory of the starting <lactol> OH. Coordination is a mechanistic explanation consistent with MgBr2 and the <product (chemistry)>; the scheme does not quantify individual organomagnesium aggregates.<Felkin-Anh and Cram Chelate by University of Windsor>[p. 3, section 2, Chelation and Addition–Cram Chelate Rule]{ref} The relevant primary synthetic sequence is documented independently.<Highly Stereoselective Asymmetric Total Synthesis of (−)-Jimenezin via Sequential Intramolecular Amide Enolate Alkylation by Thien Nhan Lu and others>[2022, synthetic sequence leading through the protected oxygen heterocycle]{ref}
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