Hi,

I configured Umbraco with a new OWIN startup like this:

using Microsoft.Owin;
using Owin;
using Umbraco.Core;
using Umbraco.Core.Security;
using Umbraco.Web;
using Umbraco.Web.Security.Identity;
using Umbraco.Core.Models.Identity;
using Umbraco.IdentityExtensions;
using DocManagement_WorkFlow;


//To use this startup class, change the appSetting value in the web.config called 
// "owin:appStartup" to be "UmbracoStandardOwinStartup"

[assembly: OwinStartup("UmbracoStandardOwinStartup", typeof(UmbracoStandardOwinStartup))]

namespace DocManagement_WorkFlow
{
    /// <summary>
    /// The standard way to configure OWIN for Umbraco
    /// </summary>
    /// <remarks>
    /// The startup type is specified in appSettings under owin:appStartup - change it to "StandardUmbracoStartup" to use this class
    /// </remarks>
    public class UmbracoStandardOwinStartup : UmbracoDefaultOwinStartup
    {
        public override void Configuration(IAppBuilder app)
        {
            //ensure the default options are configured
            base.Configuration(app);

            /* 
             * Configure external logins for the back office:
             * 
             * Depending on the authentication sources you would like to enable, you will need to install 
             * certain Nuget packages. 
             * 
             * For Google auth:                 Install-Package UmbracoCms.IdentityExtensions.Google
             * For Facebook auth:                   Install-Package UmbracoCms.IdentityExtensions.Facebook
             * For Microsoft auth:                  Install-Package UmbracoCms.IdentityExtensions.Microsoft
             * For Azure ActiveDirectory auth:      Install-Package UmbracoCms.IdentityExtensions.AzureActiveDirectory
             * 
             * There are many more providers such as Twitter, Yahoo, ActiveDirectory, etc... most information can
             * be found here: http://www.asp.net/web-api/overview/security/external-authentication-services
             * 
             * For sample code on using external providers with the Umbraco back office, install one of the 
             * packages listed above to review it's code samples 
             *  
             */

            /*
             * To configure a simple auth token server for the back office:
             *             
             * By default the CORS policy is to allow all requests
             * 
             *      app.UseUmbracoBackOfficeTokenAuth(new BackOfficeAuthServerProviderOptions());
             *      
             * If you want to have a custom CORS policy for the token server you can provide
             * a custom CORS policy, example: 
             * 
             *      app.UseUmbracoBackOfficeTokenAuth(
             *          new BackOfficeAuthServerProviderOptions()
             *              {
             *                  //Modify the CorsPolicy as required
             *                  CorsPolicy = new CorsPolicy()
             *                  {
             *                      AllowAnyHeader = true,
             *                      AllowAnyMethod = true,
             *                      Origins = { "http://mywebsite.com" }                
             *                  }
             *              });
             */

            // active directory authentication
            var applicationContext = ApplicationContext.Current;

            app.ConfigureUserManagerForUmbracoBackOffice<BackOfficeUserManager, BackOfficeIdentityUser>(
                applicationContext,
                (options, context) =>
                {
                    var membershipProvider = Umbraco.Core.Security.MembershipProviderExtensions.GetUsersMembershipProvider().AsUmbracoMembershipProvider();
                    var settingContent = Umbraco.Core.Configuration.UmbracoConfig.For.UmbracoSettings().Content;
                    var userManager = BackOfficeUserManager.Create(
                        options,
                        applicationContext.Services.UserService,
                        applicationContext.Services.EntityService,
                        applicationContext.Services.ExternalLoginService,
                        membershipProvider,
                        settingContent
                    );

                    userManager.BackOfficeUserPasswordChecker = new ActiveDirectoryBackOfficeUserPasswordChecker();
                    return userManager;
                });


        }



    }
}

and configure web.config and umbracoSettings

web.config

 <add key="ActiveDirectoryDomain" value="linde.grp" />
  <add connectionString="LDAP://x.y.z.k:389/DC=abc,DC=pippo,DC=grp" name="ADConnectionString" />

 <membership defaultProvider="UmbracoMembershipProvider" userIsOnlineTimeWindow="15">
      <providers>
        <clear />
        <add name="UmbracoMembershipProvider" type="Umbraco.Web.Security.Providers.MembersMembershipProvider, Umbraco" minRequiredNonalphanumericCharacters="0" minRequiredPasswordLength="10" useLegacyEncoding="false" enablePasswordRetrieval="false" enablePasswordReset="false" requiresQuestionAndAnswer="false" defaultMemberTypeAlias="Member" passwordFormat="Hashed" allowManuallyChangingPassword="false" />

        <add name="UsersMembershipProvider" type="Umbraco.Web.Security.Providers.UsersMembershipProvider, Umbraco" />

        <add name="BackofficeMembershipProvider" type="System.Web.Security.ActiveDirectoryMembershipProvider" connectionStringName="ADConnectionString" attributeMapUsername="sAMAccountName" connectionUsername="PIPPO\UTENTE" connectionPassword="password" />

      </providers>
    </membership>

umbracoSettings

 <providers>
    <users>
      <DefaultBackofficeProvider>BackofficeMembershipProvider</DefaultBackofficeProvider>
    </users>
  </providers>

How to authenticate to AD? I create the backoffice user with username=AD login.